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A new gas-phase infrared (IR) spectrum of acryloyl fluoride (ACRF, CH2 CH CF O) with a resolution of 0.1 cm− 1 in the range 4000–450 cm− 1 was measured. Theoretical ab initio molecular structures, full quartic potential energy surfaces (PES), and cubic surfaces of dipole moments and polarizability tensor components (electro-optical properties, EOP) of the s-trans and s-cis conformers of the ACRF were calculated by the second-order Moller-Plesset electronic perturbation theory with a correlation consistent Dunning triple-ζ basis set. The numerical-analytic implementation of the second-order operator canonical Van Vleck perturbation theory was employed for predicting anharmonic IR and Raman scattering (RS) spectra of ACRF. To improve the anharmonic predictions, harmonic frequencies were replaced by their counterparts evaluated with the higher-level CCSD(T)/cc-pVTZ model, to form a “hybrid” PES. The original operator representation of the Hamiltonian is analytically reduced to a quasi-diagonal form, integrated in the harmonic oscillator basis and diagonalized to account for strong resonance couplings. Double canonical transformations of EOP expansions enabled prediction of integral intensities of both fundamental and multi-quanta transitions in IR/RS spectra. Enhanced band shape analysis reinforced the assignments. A thorough interpretation of the new IR experimental spectra and existing matrix-isolation literature data for the mixture of two conformers of ACRF was accomplished, and a number of assignments clarified.

The ground-state rotational spectrum of propene-3-d1, CH2═CHCH2D, was measured by Fourier transform microwave spectroscopy. Transitions were assigned for the two conformers, one with the D atom in the symmetry plane (S) and the other with the D atom out of the plane (A). The energy difference between the two conformers was calculated to be 6.5 cm–1, the S conformer having lower energy. The quadrupole hyperfine structure due to deuterium was resolved and analyzed for both conformers. The experimental quadrupole coupling and the centrifugal distortion constants compared favorably to their ab initio counterparts. Ground-state rotational constants for the S conformer are 40582.157(9), 9067.024(1), and 7766.0165(12) MHz. Ground-state rotational constants for the A conformer are 43403.75(3), 8658.961(2), and 7718.247(2) MHz. For the A conformer, a small tunneling splitting (19 MHz) due to internal rotation was observed and analyzed. Using the new rotational constants of this work as well as those previously det...

The vibrational spectra of 2,3,3,3-tetrafluoropropene (2333TFP) are studied in the infrared experimentally and theoretically by the canonical second-order Van Vleck operator perturbation theory (CVPT2) and full quartic potential energy surface (PES). 2333TFP belongs to hydrofluoroolefins (HFOs) considered among the most promising alternatives to the hydrofluorocarbons presently in use in refrigeration. The medium resolution infrared spectra of gaseous 2333TFP were recorded in the range 9500–30 cm − 1 . The integrated IR intensities for all bands falling between 6300–400 cm − 1 were accurately determined. Theoretical descriptions of up to four-quanta anharmonic vibrational states of 2333TFP are performed using the state-of-the-art numerical-analytic implementation of CVPT2. High quality harmonic frequencies, obtained at different coupled-cluster levels of theory up to the CCSD(T*)-F12c/VTZ-F12 model were combined with the full quartic MP2/cc-pVTZ anharmonic force field to form the hybrid PES. All Fermi and Darling-Dennison resonances were detected using a universal criterion and treated with the variational CVPT2/VCI stage by using two-, three-, and four-quanta harmonic oscillator basis sets. General recommendations for choosing such basis sets in future studies are formulated. In addition to all fundamentals and their resonance satellites, the vibrational analysis led to the assignment of virtually all observed first/second overtone and binary/ternary combination bands. Computed dipole moment first derivatives in the principal axis system yielded reliable predictions of band shapes for fundamental transitions in good agreement with observed counterparts. Computed anharmonic IR intensities showed an excellent agreement with observed data in the measured ranges. The fundamental spectroscopic effect of excited resonances is observed experimentally and explained theoretically as a universal mechanism of formation of vibrational polyads. The efficiency of the CVPT2/VCI approach employed is clearly demonstrated and can be considered as a benchmark for modeling vibrational states and interpretation of vibrational spectra of semi-rigid molecules.

Prior to the mid-1880s aluminum was known as a metallic substance but was too costly to be used for other than jewelry-type applications. In 1886, Charles Hall in the United States and Paul Héroult in France discovered an economical electrolysis process for reducing aluminum from its abundant ore, alumina (Al2O3). This method, known today as the Hall–Héroult process, was a direct application of the then-new development of dynamos and principally of waterpower to generate huge amounts of electricity. Within a few years, aluminum was being produced at a low enough price that this metal played a growing role in everyday life. As a lustrous and lightweight metal, aluminum transformed human expectations for the appearance and uses of metals. This paper traces the stories of Hall and Héroult in their historic paths from concept to industrialization for refining aluminum metal. The essentials of the Hall–Héroult process remain fundamental in the aluminum industry today.

New measurements of microwave lines (A and E) of propene and its three 13C1 isotopologues have been made in the 10–22 GHz region with FT accuracy. The revised lines for propene along with many hundreds from the literature were fitted with the ERHAM program for internal rotors to give improved rotational constants. The new constants are A0 = 46280.2904(16), B0 = 9305.24260(30), and C0 = 8134.22685(28) MHz. Lines for the 3-13C1 species were observed in a pure sample; lines for the 1-13C1 and 2-13C1 species were observed in natural abundance. In fitting the limited sets of lines for the 13C1 species, many of the centrifugal distortion constants and most of the tunneling parameters were transferred from the fit of propene itself with 27 parameters. Improved rotational constants for the 13C1 species are reported.

IR spectra of trans -diazene (diimide, HN NH) have been recorded in nitrogen and argon matrices at 20 K. An IR spectrum of a mixture of trans -diazene- d 1 and - d 2 in a nitrogen matrix has also been recorded. The method for preparing pure trans-diazene has been clarified. Revised assignments are reported for these spectra. A definitive experimental assignment of the wavenumbers of normal modes the three hydrogen/deuterium isotopologues is given. Attempts were made to photoisomierize matrix-isolated trans -diazene into the elusive cis isomer. No bands for the cis isomer were found.

Electronic structure theory reveals that π-electron delocalization increases with the chain length in polyenes. To analyze quantitatively this effect a semiexperimental equilibrium structure has been determined for trans-hexatriene by the mixed estimation method. For this fit rotational constants for a number of carbon and hydrogen isotopologues as well as a high-level ab initio structure have been used. The accuracy is 0.001 Å for bond lengths and 0.1° for bond angles. For the three isomers of octatetraene, high-level ab initio calculations have given a comparably accurate structure. These structures have been used in comparison with the structure of s-trans-butadiene to show that "C═C" bonds increase in length and "C-C" bonds decrease in length as the polyene chain lengthens. These structural effects of π-electron delocalization increase toward the center of polyenes. Most likely, π-π conjugation in the molecules studied plays a large part in their planarity that, in turn, forces the hydrogen atoms of cis fragments in bay regions to be in a close contact. Their distance is indeed shorter than the sum of their van der Waals radii, and they seem to participate in a six-membered ring.

The 2- d 1 and 3- d 1 isotopologues of trans -hexatriene have been synthesized, and their high-resolution (0.0015 cm −1 ) IR spectra have been recorded. For each of the isotopologues the rotational structure in four C-type bands for out-of-plane vibrational modes has been analyzed, and the ground state combination differences (GSCDs) have been pooled. Ground state rotational constants have been fitted to the GSCDs. For the 2- d species, A 0 , B 0 , and C 0 values of 0.7837254(5), 0.0442806(3), and 0.0419299(2) cm −1 were fitted to 2450 GSCDs. For the 3- d species, A 0 , B 0 , and C 0 values of 0.7952226(8), 0.0446149(7), and 0.0422661(4) cm −1 were fitted to 2234 GSCDs. For the eleven out-of-plane modes of the two isotopologues, predictions of wavenumbers corrected for anharmonicity and harmonic intensities have been computed and compared with experiment where possible.

As shown by the rates of proton-deuteron exchange in ethylenes with halogen substituents, the weak acidity of vinyl CH bonds is enhanced by halogen substitution. Relative rates of exchange in basic deuterium oxide reflect the relative acidities. Substitution in the α position has the strongest effect. Less electronegative halogens such as bromine increase the acidity more than does fluorine. The vinyl CH acid strengths correlate closely with the energies of deprotonation of isolated molecules into isolated anions, as computed with the MP2/cc-pVQZ model. The smaller deprotonation energies are associated with the stronger acids. Atomic charges from a natural bond order analysis done with the MP2/aug-cc-pVQZ model show that the negative charge becomes more dispersed in the anions of the stronger acids. Results are given for 13 haloethylenes and for 6 halogen-substituted butadienes, cyclopropenes, and a cyclobutene.

Samples of cis , cis - and trans , trans -1,4-difluorobutadiene-1- d 1 and of trans , trans -1,4-difluorobutadiene-1,4- d 2 have been synthesized, and high-resolution (⩽0.0018 cm −1 ) infrared spectra of these substances have been recorded in the gas phase. Analysis of the rotational structure, mostly in C-type bands, has yielded ground state rotational constants. For the two 1-d 1 species more than one band has been analyzed. For the 1,4-d 2 species only one band was available for analysis. However, good agreement between the experimental centrifugal distortion constants and those predicted with a B3LYP/cc-pVTZ model give strong support to the analysis of the very dense spectrum. The ground state rotational constants are a contribution to finding semiexperimental equilibrium structures of the two nonpolar isomers of 1,4-difluorobutadiene.

Starting with the fundamental and general criterion for a spontaneous process in thermodynamics, ΔStot ≥ 0, we review its relationships to other criteria, such as ΔA and ΔG, that have limitations. The details of these limitations, which can be easily overlooked, are carefully explicated. We also bring in the important example of electrical energy to show how criteria for spontaneity are properly applied to electrochemical cells. The analysis in this article gains clarity from use of the “global” formulation of thermodynamics and from carrying out finite changes of thermodynamic properties rather than manipulating differential changes. Several examples are given.

Microwave transitions and ground state rotational constants are reported for five newly synthesized deuterium isotopologues of cis-1,3,5-hexatriene (cHTE). These rotational constants along with those of the parent and the three (13)C species are used with vibration-rotation constants calculated from an MP2/cc-pVTZ model to derive an equilibrium structure. That structure is improved by the mixed estimation method. In this method, internal coordinates from good-quality quantum chemical calculations (with appropriate uncertainties) are fit simultaneously with moments of inertia of the full set of isotopologues. The new structure of cHTE is confirmed to be planar and is stabilized by an interaction between the hydrogen atoms H2 and H5, which form a bond and participate in a six-membered ring. cHTE shows larger structural effects of π-electron delocalization than does butadiene with the effects being magnified in the center of the molecule. Thus, strong structural evidence now exists for an increase in π-electron delocalization as the polyene chain lengthens.

Mixtures of trans -hexatriene-1,1-d 2 , - cis -1-d 1 , and - trans -1-d 1 have been synthesized. Anharmonic frequencies and harmonic intensities were predicted with the B3LYP/cc-pVTZ model for the out-of-plane ( a ″) modes of the three isotopologues. Assignments are proposed for most of the a ″ vibrational modes above 500 cm −1 . Ground state (GS) rotational constants have been determined for the 1,1-d 2 and cis -1-d 1 species from the analysis of rotational structure of C-type bands in the high-resolution (0.0015 cm −1 ) infrared spectra in a mixture of the three isotopologues. The GS constants for the 1,1-d 2 species are A 0 = 0.8018850(6), B 0 = 0.0418540(6), and C 0 = 0.0397997(4) cm −1 . The GS constants for the cis -1-d 1 species are A 0 = 0.809388(1), B 0 = 0.043532(2), and C 0 = 0.041320(1) cm −1 . Small inertial defects confirm planarity for both species. These ground state rotational constants are intended for use in determining a semiexperimental equilibrium structure and evaluating the influence of chain length on π-electron delocalization in polyenes.

A new design for experiments in the general chemistry laboratory incorporates a "do-it-yourself" component for students. In this design, students perform proven experiments to gain experience with techniques for about two-thirds of a laboratory session and then spend the last part in the do-it-yourself component, applying the techniques to an experiment of their own design. An emphasis on classifying inorganic reactions as acid-base, redox, complexation, and precipitation supports this program. An example is an enthalpy of reaction experiment in which students study an acid-base reaction, a complexation reaction, a precipitation reaction, and a redox reaction. Students perform three of the proven reactions and then do the fourth type with a reaction of their choosing. Other examples of experiments are described. The do-it-yourself component engages students in design and interpretation within a reasonable framework. With this approach, students take chances with new ideas as do working scientists.

trans-Hexatriene-1-(13)C(1) (tHTE-1-(13)C(1)) has been synthesized, and its high-resolution (0.0015 cm(-1)) infrared spectrum has been recorded. The rotational structure in the C-type bands for ν(26) at 1011 cm(-1) and ν(30) at 894 cm(-1) has been analyzed. To the 1458 ground state combination differences from these bands, ground state rotational constants were fitted to a Watson-type Hamiltonian to give A(0) = 0.8728202(9), B(0) = 0.0435868(4), and C(0) = 0.0415314(2) cm(-1). Upper state rotational constants for the ν(30) band were also fitted. Predictions of the ground state rotational constants for tHTE-1-(13)C(1) from a B3LYP/cc-pVTZ model with scale factors based on the normal species were in excellent agreement with observations. Similar good agreement was found between predicted and observed ground state rotational constants for the three (13)C(1) isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three (13)C(1) isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C(6) backbone of tHTE. This structure shows increased structural effects of π-electron delocalization in comparison with butadiene and some differences from the cis isomer of HTE. Structures predicted with the MP2/cc-pVTZ model are also compared.

Pure samples of cis,cis - and trans,trans -1,4-difluorobutadiene-2- d 1 have been synthesized, and high-resolution (0.0015 cm −1 ) infrared spectra have been recorded for these nonpolar molecules in the gas phase. For the cis,cis isomer, the rotational structure in two C-type bands at 775 and 666 cm −1 and one A-type band at 866 cm −1 has been analyzed to yield a combined set of 2020 ground state combination differences (GSCDs). Ground state rotational constants fit to these GSCDs are A 0 = 0.4195790(4), B 0 = 0.0536508(8), and C 0 = 0.0475802(9) cm −1 . For the trans,trans isomer, three C-type bands at 856, 839, and 709 cm −1 have been investigated to give a combined set of 1624 GSCDs. Resulting ground state rotational constants for this isomer are A 0 = 0.9390117(8), B 0 = 0.0389225(4), and C 0 = 0.0373778(3) cm −1 . Small inertial defects confirm the planarity of both isomers in the ground state. Upper state rotational constants have been determined for most of the transitions. The ground state rotational constants for the two isotopologues will contribute to the data set needed for determining semiexperimental equilibrium structures for the nonpolar isomers of 1,4-difluorobutadiene.

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d 2 , 1,1,4,4-d 4 , and d 6 isotopologues have been recorded with high sensitivity and resolution of 0.7 cm −1 . Hot band series of fundamentals and combinations involving the ν 13 torsional vibration of the s-trans rotamer have been observed for each of the isotopologues. Modes studied were ν 10 (C H wag), ν 12 (CH 2 twist), ν 10 + ν 12 , ν 15 (CH 2 wag) + ν 16 (CH 2 twist), and ν 23 (CH 2 rock) + ν 24 (C C C deformation). The spacings of the quantum states of the torsional contribution were found to decrease with additional excitation of this mode ( ν 13 ) in the upper vibrational states except for the ν 23 + ν 24 combination state.

Nearly 50 years ago, Henry Bent published his groundbreaking article in this Journal introducing the “global” formulation of thermodynamics. In the following years, the global formulation was elabo...

The equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene, C(2)H(2)F(2), have been determined with high-level coupled cluster techniques combined with large basis sets, explicit consideration of core/valence, and scalar relativistic and higher order correlation effects. Excellent agreement was found with new semiexperimental structures, increasing the level of confidence in both approaches. Differences in bond lengths among ethylene and the fluoroethylenes are discussed.

Infrared (IR) spectra in the gas phase are reported for CF(2)=CHD and CF(2)=CD(2) in the region 350-4000 cm(-1). Ab initio calculations of an harmonic force-field and anharmonicity constants have been made with an MP2/aug-cc-pVTZ model. These enable a number of Fermi resonances in each species to be analyzed and a complete set of "observed" harmonic frequencies to be derived. The latter are combined with similar data for CF(2)=CH(2) in a scaling of the model harmonic force field to both anharmonic and harmonic frequencies. Inspection of the scale factors reveals minor defects of the model, evident in the out-of-plane wagging modes and in the CF stretch/CF stretch interaction force constant. Fermi resonance treatments involved in all isotopomers studied are compatible with the overall force-field refinement results. The treatment leaves a small anomaly in the (13)C shift on nu(1). Improved microwave spectra are reported for five isotopic species, and a semiexperimental equilibrium structure for F(2)C=CH(2) is determined and compared favorably with the structure obtained from new high-level ab initio calculations. Centrifugal distortion constants are predicted for the five isotopic species, and those for F(2)C=CH(2) are compared with values fit to microwave spectra.

Assignments of the vibrational fundamentals of cis - and trans -1,3,5-hexatriene are reevaluated with new infrared and Raman spectra and with quantum chemical predictions of intensities and anharmonic frequencies. The rotational structure is analyzed in the high-resolution (0.0013–0.0018 cm −1 ) infrared spectra of three C-type bands of the trans isomer and two C-type bands of the cis isomer. The bands for the trans isomer are at 1010.96 cm −1 ( ν 14 ), 900.908 cm −1 ( ν 16 ), and 683.46 cm −1 ( ν 17 ). Ground state (GS) rotational constants have been fitted to the combined ground state combination differences (GSCDs) for the three bands of the trans isomer. The bands for the cis isomer are at 907.70 cm −1 ( ν 33 ) and 587.89 cm −1 ( ν 35 ). GS rotational constants have been fitted to the combined GSCDs for the two bands of the cis isomer and compared with those obtained from microwave spectroscopy. Small inertial defects in the GSs confirm that both molecules are planar. Upper state rotational constants were fitted for all five bands.

Three dihalogenated methane derivatives (CH2F2, CH2FCl, and CH2Cl2) were used as model systems to compare and assess the accuracy of two different approaches for predicting observed fundamental frequencies: canonical operator Van Vleck vibrational perturbation theory (CVPT) and vibrational configuration interaction (VCI). For convenience and consistency, both methods employ the Watson Hamiltonian in rectilinear normal coordinates, expanding the potential energy surface (PES) as a Taylor series about equilibrium and constructing the wavefunction from a harmonic oscillator product basis. At the highest levels of theory considered here, fourth-order CVPT and VCI in a harmonic oscillator basis with up to 10 quanta of vibrational excitation in conjunction with a 4-mode representation sextic force field (SFF-4MR) computed at MP2/cc-pVTZ with replacement CCSD(T)/aug-cc-pVQZ harmonic force constants, the agreement between computed fundamentals is closer to 0.3 cm-1 on average, with a maximum difference of 1.7 cm-1. The major remaining accuracy-limiting factors are the accuracy of the underlying electronic structure model, followed by the incompleteness of the PES expansion. Nonetheless, computed and experimental fundamentals agree to within 5 cm-1, with an average difference of 2 cm-1, confirming the utility and accuracy of both theoretical models. One exception to this rule is the formally IR-inactive but weakly allowed through Coriolis-coupling H-C-H out-of-plane twisting mode of dichloromethane, whose spectrum we therefore revisit and reassign. We also investigate convergence with respect to order of CVPT, VCI excitation level, and order of PES expansion, concluding that premature truncation substantially decreases accuracy, although VCI(6)/SFF-4MR results are still of acceptable accuracy, and some error cancellation is observed with CVPT2 using a quartic force field.

High-resolution (0.0013 cm −1 ) infrared spectra have been recorded for trans,trans -1,4-difluorobutadiene (ttDFBD) and cis,cis- 1,4-difluorobutadiene (ccDFBD). The rotational structure in two C-type bands ( ν 10 and ν 12 ) and one A-type band ( ν 22 ) for ttDFBD and in two C-type bands ( ν 11 and ν 12 ) for ccDFBD has been analyzed. Ground state and upper state rotational constants, except for ν 10 of ttDFBD, have been fitted. Band centers are 934.1 cm −1 ( ν 10 ), 227.985 cm −1 ( ν 12 ), and 1087.919 cm −1 ( ν 22 ) for ttDFBD. Band centers are 762.891 cm −1 ( ν 11 ) and 327.497 cm −1 ( ν 12 ) for ccDFBD. The small inertial defects in the ground state confirm that both isomers are planar. Obtaining the ground state rotational constants for the two isomers of DFBD is a first step toward determining their semi-experimental equilibrium structures.

High level ab initio calculations, utilizing coupled cluster theory with quasi-perturbative triple excitations and augmented quadruple zeta level basis sets, have been used to determine the structures and relative energies of the four stationary points on the 1,3-butadiene torsional potential curve. Corrections were applied in order to minimize the residual basis set error, as well as account for core/valence correlation and scalar relativistic effects. Higher order correlation recovery was also included to improve our estimate of the relative energies. The transition state separating the trans and gauche rotamers lies 26.8 kJ/mol above the trans global minimum. The gauche rotamer lies 12.6 kJ/mol above the trans rotamer and the s-cis form is a transition state 2.0 kJ/mol higher than the gauche rotamer (excluding zero point energies).

Twenty-five microwave lines were observed for cis-1,3,5-hexatriene (0.05 D dipole moment) and a smaller number for its three (13)C isotopomers in natural abundance. Ground-state rotational constants were fitted for all four species to a Watson-type rotational Hamiltonian for an asymmetric top (kappa = -0.9768). Vibration-rotation (alpha) constants were predicted with a B3LYP/cc-pVTZ model and used to adjust the ground-state rotational constants to equilibrium rotational constants. The small inertial defect for cis-hexatriene shows that the molecule is planar, despite significant H-H repulsion. The substitution method was applied to the equilibrium rotational constants to give a semiexperimental equilibrium structure for the C(6) backbone. This structure and one predicted with the B3LYP/cc-pVTZ model show structural evidence for increased pi-electron delocalization in comparison with butadiene, the first member of the polyene series.

The rotational spectra of eight isotopomers of 1-chloro-1-fluoroethylene in the 6–22 GHz region have been collected and analyzed. Each rotational transition is split into hyperfine components by the chlorine (either 35 Cl or 37 Cl) nuclear quadrupole coupling interaction and additionally, one or more smaller interactions such as the spin–rotation interaction due to the fluorine atom, hydrogen–hydrogen spin–spin coupling interactions, and in appropriately substituted species, the deuterium nuclear quadrupole hyperfine interaction. The rotational constants derived from these isotopomers allow the determination of average and Kraitchman substitution structures for 1-chloro-1-fluoroethylene, whereas the availability of the diagonal chlorine nuclear quadrupole coupling constants for all the isotopomers provides complete quadrupole coupling tensors for both 35 Cl and 37 Cl. In the course of this work, the rotational spectrum of an excited vibrational state of the normal isotopomer was observed, which ab initio calculations suggest should be assigned to ν 9 = 1 , an in-plane bending motion at the CFCl end of the molecule.

Anharmonic and related constants have been calculated for CH2Cl2, CD2Cl2, and CHDCl2 by using the program Gaussian03 and B3LYP and MP2 models. Bases used were 6-311++G** and cc-pVTZ. The size of grid used in the B3LYP/6-311++G** model had a noticeable effect on resulting data. Features of the MP2/6-311++G** calculations suggested a deleterious effect of the absence of f functions in this basis set. The need for the replacement of second-order terms in the perturbation theory formulas for the vibrational anharmonic constants x ij in the presence of Fermi resonance was explored, and minor resonances were found associated with the cubic constants varphi 122 and varphi 299 (d 0 isotopomer), phi122 and phi849 (d2), and phi278 (d1). Computed xij values for nuCH and nuCD motions agree quite well with earlier experimental data. Observed anharmonic frequencies, nu obsd, were corrected to "observed" harmonic frequencies, omega obsd, by using computed differences Delta = omegaQC-nuQC. These differences Delta are larger for the antisymmetric nuasCH2 mode than for symmetric nusCH2 motion. This fact made it necessary to use differing scale factors for the two kinds of CH stretching force constants in a subsequent scaling of the harmonic force field to nuobsd. Force field scaling was also carried out by refining to omega obsd. In both approaches, the B3LYP models required differing scale factors for symmetric and antisymmetric CCl stretching force constants, indicating a failure to compute an accurate C-Cl stretch-stretch interaction force constant. The MP2/cc-pVTZ force field was preferred. Both scaled and unscaled harmonic force fields were used to calculate centrifugal distortion constants (CDCs) and contributions to the vibrational dependence of the rotational constants (alphas). Variations in the CDCs can, in part, be explained by the magnitudes of the frequencies used in the scaling process.

Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 A, and the formal C-C bond length is 1.454 A. For ethylene, the C=C bond length is 1.3305 A. These values appear to be good to 0.001 A. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 A and decreasing the length of the formal single bond by 0.016 A. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.

The 2,3-13C2 isotopomer of butadiene was synthesized, and its fundamental vibrational fundamentals were assigned from a study of its infrared and Raman spectra aided with quantum chemical predictions of frequencies, intensities, and Raman depolarization ratios. For two C-type bands in the high-resolution (0.002 cm−1) infrared spectrum, the rotational structure was analyzed. These bands are for ν11 (au) at 907.17 cm−1 and for ν12 (au) at 523.37 cm−1. Ground state and upper state rotational constants were fitted to Watson-type Hamiltonians with a full quartic set of centrifugal distortion constants and two sextic ones. For the ground state, A0 = 1.3545088(7) cm−1, B0 = 0.1469404(1) cm−1, and C0 = 0.1325838(2) cm−1. The small inertial defects of butadiene and two 13C2 isotopomers, as well as for five deuterium isotopomers as previously reported, confirm the planarity of the s-trans rotamer of butadiene.

The distinction between system-based and surroundings-based definitions of thermodynamic heat ( q ) and work ( w ), particularly pressure–volume work, in irreversible processes is introduced and cleanly drawn. A systematic presentation of system-based q and w is given for the first time. This development complements the authors’ earlier presentation of surroundings-based work and heat. Either set of definitions can and has been used to develop the laws of thermodynamics. Both sets of definitions are used to analyze examples presented by Kivelson and Oppenheim (KO). It is seen for two KO processes that w (sys-based) and w (surr-based) are not equal. This not uncommon result does not violate the first law because the two q values are also different. One of the KO examples corresponds to a “quasistatic” process, which is reversible from the point of view of the system but not from the point of view of the system plus surroundings taken together, and, therefore, not reversible overall. A number of reasons are given for preferring surroundings-based definitions of w and q to system-based definitions. Perhaps the most important is the fact that w (sys-based) does not always satisfy the theorem of maximum work in a constant temperature process. Finally, an explanation is presented for why the common use of two different sets of definitions for w and q has not led to greater confusion in the past.

The infrared spectrum of the ν 11 ( a u ) out-of-plane CH 2 -wagging vibrational mode of the environmentally important 1,3-butadiene molecule has been investigated at sub-Doppler resolution with a CO 2 -laser/microwave-sideband spectrometer in order to fully resolve the rotational structure in a number of compact R Q-branch heads. The center frequencies of over 90 saturation Lamb dips in the 11 μm region have been measured with an estimated absolute accuracy of 200 kHz. The new data have been combined with previous Fourier transform results to refine the parameters in the molecular Hamiltonian. A value of |d μ c /d Q 11 |=0.3135 debye has been obtained for the transition dipole derivative for the ν 11 mode on the basis of ab initio calculations.

Butadiene-1- 13 C 1 was synthesized, and its high-resolution (0.002 cm −1 ) infrared spectrum was recorded for several bands in the mid-infrared region. A complete analysis of the rotational structure in the C-type band at 524.485 cm −1 for CH 2 twisting and a partial analysis of the rotational structure in the C-type bands at 900.0 and 909 cm −1 were performed. Of these latter two bands, which are of comparable intensity, the higher frequency one is largely CH 2 out-of-plane wagging and the lower frequency one is largely 13 CH 2 out-of-plane wagging. Taken together these bands correlate with one infrared-active a u fundamental and one Raman-active b g fundamental of butadiene. The ground state rotational constants are A =1.3887919 (6), B =0.1436683 (3), and C =0.1302251 (3) cm −1 , and upper state rotational constants are reported for the bands at 524.485 and 900.0 cm −1 . Medium resolution infrared and Raman spectra gave a complete assignment of the vibrational fundamentals, including 11 fundamentals observed directly for the first time.

Samples of trans , trans and cis , cis forms of butadiene-1,4- d 2 have been synthesized and found to contain useful amounts of the cis , trans species as a contaminant. Assignments of fundamental frequencies for the three isotopomers of butadiene-1,4- d 2 have been extended and improved from investigations of their Raman spectra as well as their infrared (IR) spectra. High-resolution IR spectra have been recorded for the three isotopomers, and a rotational analysis has been completed for strong bands of each species. Ground state and some upper state rotational constants have been fit. Corresponding ground state moments of inertia compare favorably with equilibrium moments of inertia obtained from B3LYP/6-311++G** theory. Two 13 C isotopomers are being prepared, and an improved structural analysis of butadiene will soon be available to assess how π-electron delocalization affects its structure.

Rotational constants for butadiene and butadiene-2,3-d2 are derived from the analysis of the rotational structure of bands in the high-resolution infrared (IR) spectra. Corresponding moments of inertia computed from three quantum chemical calculations agree acceptably with the experimental values. Evidence is growing that the CC bond lengthens some and the C–C bond shortens some in support of partial π-electron delocalization in butadiene. Refinements are made in the assignment of vibrational fundamentals of butadiene and butadiene-2,3-d2 from new observations of IR and Raman spectra of these species.

Microwave spectra for seven isotopomers of cis-1,2-difluoroethylene have been reinvestigated and extended with the pulsed-molecular-beam Fourier transform method. Rotational Hamiltonians that include a full set of quartic centrifugal distortion constants have been fit for each isotopomer. Ground-state rotational constants for the trans isomer were available from an investigation of the high-resolution infrared spectra of several isotopomers. Equilibrium rotational constants have been extracted for the cis and trans isomers from the experimental ground-state constants and the vibration-rotation constants computed at the MP2/6-31G(d) level. Equilibrium structures have been fit to the equilibrium principal moments of inertia for both isomers, and isomeric differences in the bond lengths and bond angles have been interpreted in terms of the strong influence of fluorine substitution. For cis,trans-1,4-difluorobutadiene a similar derivation of the equilibrium structure has been achieved through the combination of published microwave data and quantum chemical calculations. This substance provides an opportunity to examine the influence of cis and trans fluorine substitution within a single molecule. Evidence is weighed for the adequacy of vibration-rotation constants computed with the MP2/6-31G(d) model.

Author Institution: Department of Chemistry, Oberlin College; Department of Chemistry, Kent, State University; Department of Partment of Chemistry, University of Michigan

Infrared and Raman spectra are reported for cis and trans 1,2- 1 3 C 2 H 2 F 2 . Quantum chemical calculations at B3LYP and MP2 levels have been made for cis and trans 1,2-difluoroethylenes, using 6-311G** and 6-311++G** bases. The resulting harmonic force fields for each compound were scaled with nine independent factors, using frequency data corrected, where necessary, for Fermi resonances and for liquid/gas shifts. The previously accepted assignments for v 7 and v 1 2 in the trans isomer are interchanged. Several scale factors for bending motions differ markedly between the cis and trans compounds. Centrifugal distortion constants observed with significant accuracy are predicted within 6%. Harmonic contributions to the vibrational dependence of the rotational constants and perpendicular amplitudes are calculated. The C=C and C-H bonds in the cis and trans isomers are essentially identical in respect to length, force constant, and isolated CH stretching frequency. However, the C-F bond is slightly stronger in the cis compound. A revised allocation of observed infrared intensity between the overlapping trans v 7 and v 1 2 bands is needed. Directions of ∂p/∂Q for the trans B u bands are given, which differ from those reported earlier. Magnitudes and directions of the bond dipole derivatives ∂μ/∂r for the CH and CF bonds are obtained from calculated atomic polar tensors. In both cis and trans isomers, the vector ∂μ/∂r C F lies within 17° of the bond direction, but for the CH bond, ∂μ/∂r is roughly perpendicular to the CH direction.

Microwave spectra in the 5.5-17.5-GHz region have been observed for cis,trans-1,4-difluorobutadiene, for its 1 3 C isotopomers in natural abundance, for the four singly substituted deuterium isotopomers, and for the 1,4-d 2 species. For the parent species fitting 37 lines to a Watson-type Hamiltonian with the five quartic centrifugal distortion constants gave A = 12 988.333(1). B = 1467.8791(3), and C = 1318.5845(3) MHz. Stark effect measurements on three transitions of the parent species gave a dipole moment of 2.309(5) D with components μ a = 0.660(4) D and μ b = 2.213(5) D. Surprisingly, the B rotational constant for the 2-d 1 isotopomer, as indexed from the trans end, is larger than that for the normal species, and the rotational constants for the 3-d 1 isotopomer are also anomalous. Nonetheless, a complete, 17-parameter structure is proposed for this planar molecule and compared with density functional theory predictions made with the adiabatic connection method. Structural parameters are also compared with those of butadiene itself.

Extended and improved microwave (MW) measurements are reported for the isotopologues of piperidine. New ground state (GS) rotational constants are fitted to MW transitions with quartic centrifugal distortion constants taken from ab initio calculations. Predicate values for the geometric parameters of piperidine and cyclohexane are found from a high level of ab initio theory including adjustments for basis set dependence and for correlation of the core electrons. Equilibrium rotational constants are obtained from GS rotational constants corrected for vibration-rotation interactions and electronic contributions. Equilibrium structures for piperidine and cyclohexane are fitted by the mixed estimation method. In this method, structural parameters are fitted concurrently to predicate parameters (with appropriate uncertainties) and moments of inertia (with uncertainties). The new structures are regarded as being accurate to 0.001 Å and 0.2°. Comparisons are made between bond parameters in equatorial piperidine and cyclohexane. Another interesting result of this study is that a structure determination is an effective way to check the accuracy of the ground state experimental rotational constants.

The complete molecular structure of the high-energy gauche rotamer of 1,1,2,2-tetrafluoroethane (TFEA), Freon F134, has been determined using pulsed-molecular-beam Fourier transform microwave spectroscopy. Five different 13C- and 2H-substituted isotopomers, in addition to the normal species, were studied. Unlike the nonpolar anti rotamer, the gauche form has a permanent electric dipole moment, which permits a direct rotational analysis. The electric dipole moment was also determined using the Stark effect of two low-J transitions. The resulting dipole moment is μc = 8.186(7) × 10-30 C m [2.454(2) Debye] where the numbers in parentheses are the standard uncertainties (i.e., 1σ). The experimental results for the geometric parameters of TFEA are compared with two recent ab initio calculations.

The 13C2 and d2 isotopomers of 1,1,2,2-tetrafluoroethane (TFEA) have been synthesized. Raman spectra of these new species have been recorded, and infrared spectra of all three isotopomers, including some regions with high-resolution at −100 °C, have also been recorded. Guided by recently published calculations of frequencies and infrared intensities and the new spectra, we have revised the previous assignments of fundamentals for the two rotamers of the normal species of TFEA. Assignments of the fundamentals for both rotamers of the 13C2 and d2 isotopomers are proposed. The anti rotamer is the more abundant species in the gas phase and, to a lesser extent, in the liquid phase and the only species in the crystal phase. Thus, the assignments of the anti rotamer of all three isotopic species are complete and supported by isotope product rules, but the assignments for the gauche rotamers are incomplete. Estimates of the missing frequencies for the gauche rotamer of the normal species are supplied.

Infrared and Raman spectra were recorded for the trans,trans (EE), cis,cis (ZZ), and cis,trans (ZE) isomers of 1,4-difluorobutadiene (DFBD). From these spectra and frequencies predicted from the adiabatic connection method, which is a hybrid of Hartree−Fock and density−functional theories, complete assignments of fundamentals were made for the observable s-trans configurations. The fundamentals for the trans,trans isomer are (in cm-1): (ag) 3091, 3048, 1681, 1325, 1280, 1151, 1121, 409, 383; (au) 934, 798, 227, 154; (bg) 897, 830, 397; and (bu) 3086, 3056, 1638, 1299, 1221, 1088, 621, 133. The fundamentals for the cis,cis isomer are (in cm-1): (ag) 3118, 3088, 1676, 1410, 1248, 1134, 946, 751, 232; (au) 914, 762, 330, 78; (bg) 897, 789, 580; and (bu) 3109, 3092, 1624, 1340, 1215, 1044, 632, 165. The fundamentals for the cis,trans isomer are (in cm-1): (a‘) 3114, 3082, 3062, 3036, 1690, 1629, 1391, 1313, 1253, 1224, 1138, 1129, 1008, 706, 504, 308, 138; and (a‘ ‘) 929, 887, 824, 758, 526 (calculated), 2...