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The possibility of extending the relationships between density, binding forces, and bonding energies to fine chemical effects is tested taking as an example the rotational barrier of ethane. Electron densities that reasonably fulfill the electrostatic theorem were obtained for several conformations using a Slater basis set. The analysis of these densities shows that the barrier is due to the internal forces acting on the H nuclei. Out of the staggered and eclipsed conformations, the clouds of the hydrogen atoms have a nonskeletal distortion that pull their nuclei toward the staggered conformation.

We report a two dimensional, laser induced fluorescence study of the lowest 345 cm(-1) region of S0 toluene. Methyl rotor levels of 00 up to m = 6 and of 201 up to m = 4 are observed. The rotor levels of 00 and 201 have quite different energy spacings that are well fit by a model that includes strong torsion-vibration coupling between them. The model requires that the rotor barrier height be revised from -4.84 cm(-1) (methyl hydrogens in a staggered conformation) to +1.57 cm(-1) (eclipsed conformation). However, the 3a2″ state lies below the 3a1″ state as expected for a staggered conformation due to energy shifts associated with the torsion-vibration coupling. It is shown that the rotor wave-functions exhibit little localization at the torsional energy minima. The variation in the m = 0 wavefunction probability distribution with torsional angle is shown to be very similar for the previously accepted negative V6 value and the torsion-vibration coupling model as this coupling shifts the phase of the wavefunction by 30° compared with its phase for V6 alone. The presence of a strong Δυ = ± 1 torsion-vibration coupling involving the lowest frequency vibrational mode provides a potential pathway for rapid intramolecular vibrational energy redistribution at higher energies.

The possibility of electron binding to the complex (H3BNH3) was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations. The staggered conformation (minimum), with a dipole moment of 5.3 Debye, binds an electron by 984 cm−1, whereas the eclipsed conformer (saddle point) possesses a larger dipole moment (5.5 Debye) and binds an electron by 1014 cm−1. The neutral parent of the (H3BNH3)− anion involves a dative bond that is responsible for a significant polarization of the neutral species and generates a significant dipole moment.

Resonant two‐photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure S1–S0 and cation–S1 spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP(phenylsilane)=73 680±5 cm−1, IP(phenylsilane ⋅Ar)=73 517±5 cm−1 and IP(phenylsilane ⋅Ar2)=73 359±5 cm−1. We assign many low lying torsion–vibration levels of the S1 (A 1A1) state and of X 2B1 of phenylsilane+. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V6 are, in S1, B=2.7±0.2 cm−1 and V6=−44±4 cm−1; in the cation, B=2.7±0.2 cm−1 and V6=+19±3 cm−1. The sign of V6 and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a‘1 and 3a‘2 torsional levels. In S1 the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose ab...

The laser induced fluorescence excitation and single vibronic excitation dispersed fluorescence spectra have been studied for supersonic jet cooled 1-methyl-2(1h)-pyridone. The methyl torsional bands and some low frequency vibrational transitions were assigned for both ground and excited states. The torsional parameters V-3=244 cm(-1) and V-6=15 cm(-1) for the ground state and V-3 = 164 cm(-1) and V-6= 40 cm(-1) for the excited state were obtained. To get the insight into the methyl torsional barrier, ab initio calculations were performed and compared with the experimental results. Origin of potential barrier was traced by partitioning the barrier energy into changes in bond antibond interaction, structural, and steric energies accompanying methyl rotation using natural bond orbital analysis. The role of local interactions in ascertaining the barrier potential reveals that its nature cannot be understood without considering the molecular flexing. The hyperconjugation between CH sigma* and ring pi* observed in lowest unoccupied molecular orbital (LUMO) stabilizes the methyl group conformer that undergoes a 60 degrees rotation in the excited state with respect to that of the ground state, and it is the change in LUMO that plays important role in the excited state barrier formation. (c) 2005

Two possible conformers of the dinuclear complex Mn Re(CO)(10), each of C-4v symmetry, with eclipsed and staggered conformations, have been analyzed theoretically. Using both the B3LYP and BP86 density functionals we find that the staggered form is lower in energy. A determination of the B3LYP potential energy surface as a function of the Mn-Re distance is presented for both conformers. The computed bond lengths, bond angles, and rotational constant for the staggered conformation compare favorably with the results from microwave experiments. The harmonic frequencies for the staggered structure have been determined using several basis sets, with both analytical and finite difference methods. These unscaled vibrational frequencies, together with their intensities for both infrared and Raman activity, are used to assign the three most intense experimental IR and Raman bands, and in particular, the nu(CO) region. The lowest A(2) vibration was calculated to occur at 41 cm-1 in the staggered conformer; this frequency becomes imaginary in the (saddle point) eclipsed form. Several fundamentals remain to be observed experimentally. (C) 2004 American Institute of Physics.

Previous state-selected spectra of methanol in the 5ν1 OH stretch overtone region [O. V. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys. 110 (23), 11346 (1999)] revealed structure indicating intramolecular vibrational redistribution (IVR) on three timescales. Whereas in that work, methanol in the 5ν1 bright state was prepared close to the staggered conformation, methanol in the “partially eclipsed” conformation is prepared here by double resonance excitation through a torsionally excited intermediate state. The excited molecules are detected by infrared laser assisted photofragment spectroscopy (IRLAPS). In partially eclipsed methanol, the strong coupling of the ν1 OH stretch to the ν2 CH stretch becomes weaker, but the coupling responsible for the widths of the narrowest features becomes stronger.

We report a theoretical account on the static and dynamic aspects of the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the ground and first excited electronic states of the ethane radical cation. The findings are compared with the experimental photoionization spectrum of ethane. The present theoretical approach is based on a model diabatic Hamiltonian and with the parameters derived from ab initio calculations. The optimized geometry of ethane in its electronic ground state (1A1g) revealed an equilibrium staggered conformation belonging to the D3d symmetry point group. At the vertical configuration, the ethane radical cation belongs to this symmetry point group. The ground and low-lying electronic states of this radical cation are of 2Eg, 2A1g, 2Eu, and 2A2u symmetries. Elementary symmetry selection rule suggests that the degenerate electronic states of the radical cation are prone to the JT distortion when perturbed along the degenerate vibrational modes of eg symmetry. The 2A1g state is estimated to be approximately 0.345 eV above the 2Eg state and approximately 2.405 eV below the 2Eu state at the vertical configuration. The symmetry selection rule also suggests PJT crossings of the 2A1g and the 2Eg electronic states of the radical cation along the vibrational modes of eg symmetry and such crossings appear to be energetically favorable also. The irregular vibrational progressions, with numerous shoulders and small peaks, observed below 12.55 eV in the experimental recording are manifestations of the dynamic (E x e)-JT effect. Our findings revealed that the PJT activity of the degenerate vibrational modes is particularly strong in the 2Eg-2A1g electronic manifold which leads to a broad and diffuse structure of the observed photoelectron band.

Gaseous cobaltocene (CoCp2) has been investigated by electron diffraction at a nozzle‐tip temperature of about 120 °C. As expected, the molecule has the sandwichlike structure previously found for ferrocene (FeCp2) and nickelocene (NiCp2). The data show that any distortion from D5h or D5d molecular symmetry arising from the degenerate electronic ground state (Jahn–Teller effect) is small. Accordingly, the structure was refined in terms of three types of symmetric models: one with an eclipsed conformation of the rings (D5h), one with a staggered conformation (D5d), and one with freely rotating rings. The best agreement was obtained with the freely rotating ring models. However, the differences are small and it is clear that if large‐amplitude torsional motion of the rings had been included in the D5h and D5d models, the agreement from them would have been about equally good. Based on the assumption that the equilibrium conformation of CoCp2 can be approximated by one of these very symmetric models, we conc...

Results of Monte Carlo simulations of liquid ethane are presented for two thermodynamic states: (1) T=105 K, ρ=0.630 g/cc; and (2) T =181 K, ρ=0.548 g/cc. Three types of interaction models were used. The first was a two‐site model of ethane using exp‐6 type interactions with various locations considered for the interaction sites and correlation sites. The second was an eight‐site model considering all atoms locked in the staggered conformation, with geometric and potential parameters that have been derived from other data. The final model was similar to the second except that a hindered rotation of the methyl groups around the C–C bond was allowed. The Monte Carlo results demonstrate the importance of using a detailed molecular model on liquid structure. Further, the lack of importance of intramolecular rotation in liquid ethane is demonstrated.

The structure of difluorophosphine borane (HF2P–BH3) has been determined from the microwave spectra of six isotopic species. The expected staggered conformation and Cs symmetry have been confirmed. The structural parameters are: d(PH) = 1.409 ± 0.004 A; d(PF) = 1.552 ± 0.006 A; d(PB) = 1.832 ± 0.009 A; d(BHa) = 1.226 ± 0.005 A; d(BHs) = 1.200 ± 0.007 A; ∠HaBHa = 112.7 ± 0.5°; ∠HaBHs = 115.9 ± 0.4°; ∠FPF = 100.0 ± 0.5°; ∠FPH = 98.62 ± 0.25°; ∠BPF = 117.7 ± 0.3°. Two angles (PBHs = 109.9 ± 0.3°; PBHa = 99.89 ± 0.26°) indicate a tilt of the borane group away from the fluorine atoms. The value of the dipole moment obtained from Stark splittings is 2.504 ± 0.030 D. A range for the barrier to internal rotation is estimated as 3600–4500 cal/mole. The change in structural parameters of HF2PBH3 compared to similar compounds, the stability of HF2PBH3 towards dissociation, and the tilted borane group are discussed.

Infrared and Raman spectra are reported for the acid fluoride, chloride, and bromide derivatives of chloroacetic acid. Data from vapor, liquid, and solid spectra are given. The results show that these acid halides exist in the vapor and liquid states as a mixture of two isomers, only one of which remains in the solid state. The more stable isomer was found to have the trans conformation. Calculations of the normal vibrations using a Urey–Bradley potential function serve to indicate that the other isomer has a staggered conformation with an azimuthal angle close to 180°. The measured enthalpy differences (H°trans − H°staggered) between the isomers in the vapor state were − 0.76 ± 0.2 kcal/mole, − 0.97 ± 0.2 kcal/mole, and − 1.86 ± 0.27 kcal/mole for the acid fluoride, chloride, and bromide, respectively.

Infrared and Raman spectra have been recorded for fluoroacetyl fluoride, chloride, and bromide. The data show that each compound exists as an equilibrium mixture of two isomers in the vapor and liquid states. Normal coordinate analysis calculations based on a Urey–Bradley potential function indicate that the two preferred conformations are a trans and a staggered conformation which is close to the cis. In all three compounds the trans isomer is found to be the more stable in both liquid and vapor phases, with its stability being relatively diminished in the vapor state. Approximate, but nearly complete vibrational frequency assignments are made for each isomer. The vapor phase enthalpy difference ΔH° = (H°trans − H°staggered) was measured as − 2.05 ± 0.25 kcal/mole for fluoroacetyl bromide.

The structure of chloropicrin, an unstable compound, has been studied by electron diffraction, using the sector‐microphotometer method. Final parameters have been obtained by means of a least‐squares fit of a structural model to the experimental intensity curve. The structure is similar to those found previously for fluoropicrin and bromopicrin. The −CCl3 group and the N–O bond have the expected values for bond lengths and angles. However, the C–N distance is very long, 1.594 A; the ONO angle is 131.7°; the −CNO2 group is not planar; and the molecule assumes a staggered conformation similar to that found in ethane compounds with one position vacant. On the basis of a potential function of the form V=12V0(1−Acos3φ−Bcos6φ), where 0.75≤A≤1.0 and A+B=1, a barrier of 2.70–3.35 kcal/mole hindering free rotation about the C–N bond has been found.