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The total partition functions Q(T) and their first two moments QQ(T) and Q"(T), together with the isobaric heat capacities Cp(T), are computed a priori for three major MgH isotopologues on the temperature range of T = 100-3000 K using the recent highly accurate potential energy curve, spin-rotation, and non-adiabatic correction functions of Henderson et al. [J. Phys. Chem. A 117, 13373 (2013)]. Nuclear motion computations are carried out on the ground electronic state to determine the (ro)vibrational energy levels and the scattering phase shifts. The effect of resonance states is found to be significant above about 1000 K and it increases with temperature. Even very short-lived states, due to their relatively large number, have significant contributions to Q (T) at elevated temperatures. The contribution of scattering states is around one fourth of that of resonance states but opposite in sign. Uncertainty estimates are given for the possible error sources, suggesting that all computed thermochemical properties have an accuracy better than 0.005% up to 1200 K. Between 1200 and 2500 K, the uncertainties can rise to around 0.1%, while between 2500 K and 3000 K, a further increase to 0.5% might be observed for Q"(T) and Cp(T), principally due to the neglect of excited electronic states. The accurate thermochemical data determined are presented in the supplementary material for the three isotopologues of 24MgH, 25MgH, and 26MgH at 1 K increments. These data, which differ significantly from older standard data, should prove useful for astronomical models incorporating thermodynamic properties of these species. [ABSTRACT FROM AUTHOR]

The high accuracy ab initio adiabatic potential energy surfaces (PESs) of the ground electronic state of the water molecule, determined originally by Polyansky et al. [Science 299, 539 (2003)] and called CVRQD, are extended and carefully characterized and analyzed. The CVRQD potential energy surfaces are obtained from extrapolation to the complete basis set of nearly full configuration interaction valence-only electronic structure computations, augmented by core, relativistic, quantum electrodynamics, and diagonal Born-Oppenheimer corrections. We also report ab initio calculations of several quantities characterizing the CVRQD PESs, including equilibrium and vibrationally averaged (0 K) structures, harmonic and anharmonic force fields, harmonic vibrational frequencies, vibrational fundamentals, and zero-point energies. They can be considered as the best ab initio estimates of these quantities available today. Results of first-principles computations on the rovibrational energy levels of several isotopologues of the water molecule are also presented, based on the CVRQD PESs and the use of variational nuclear motion calculations employing an exact kinetic energy operator given in orthogonal internal coordinates. The variational nuclear motion calculations also include a simplified treatment of nonadiabatic effects. This sophisticated procedure to compute rovibrational energy levels reproduces all the known rovibrational levels of the water isotopologues considered, H216O, H217O, H218O, and D216O, to better than 1 cm-1 on average. Finally, prospects for further improvement of the ground-state adiabatic ab initio PESs of water are discussed. [ABSTRACT FROM AUTHOR]

The structures and the vibrational dynamics of the complexes HHe are investigated experimentally (via mass spectrometry (MS)) and at high levels of electronic-structure theory. The MS measurements reveal interesting trends about the stability of the starting members of the HHe family. The computations establish that the basically linear, strongly bound, symmetric triatomic molecular ion He(H)He, with an equilibrium H–He distance of 0.925 Å and about 2/3 but at least 1/2 of the positive charge on H, is the molecular core of all of the complexes. Definitive quantum-chemical results are obtained for HHe and HHe , including the proton affinity of He (computed to be cm−1 via the focal-point analysis (FPA) scheme), the FPA isomerisation energy between the two linear isomers of HHe (cm−1), and the dissociation energy of the HHe HHe + He reaction, with an FPA estimate of cm−1. The structural isomers of the He-solvated complexes are discussed up to n=18. A useful notation, [k−l−m]-HHe , is introduced to characterise qualitatively the three possible belts around the He–H –He core in HHe (), where l denotes the number of He atoms in the central belt and denote the number of He atoms in the top and bottom belts. Capping He atoms attached to the belts can be indicated by sub- and superscripts. Several possible indicators of microscopic superfluidity are investigated: He evaporation energies, rotational constants, and vibrational fundamentals. [ABSTRACT FROM AUTHOR]

The [FHCl]- molecular anion has been investigated in detail by means of state-of-the-art ab initio electronic structure methods, including restricted Hartree–Fock (RHF), Mo\ller–Plesset perturbation theory (MP2–MP4), and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. The one-particle Gaussian basis sets ranged in quality from F[6s4p2d], Cl[10s7p2d], and H[4s2p] to F[18s13p6d4f], Cl[20s14p7d5f], and H[8s3p2d1f]. The first phase of the investigation focused on the prediction of thermochemical, spectroscopic, and bonding properties of [FHCl]- and the chemical interpretation thereof.The final proposals for the geometric structure and binding energy of the complex are re(H–F)=0.963±0.003 Å, Re(H–Cl)=1.925±0.015 Å, and D0(HF+Cl-)=21.8±0.4 kcal mol-1. A Morokuma decomposition of the ion-molecule bonding gave the following electrostatic (ES), polarization (PL), exchange repulsion (EX), dispersion (DISP), and charge-transfer plus higher-order mixing (CT+MIX) components of the vibrationless complexation energy: -27.3 (ES), -5.2 (PL), +18.3 (EX), -4.5 (DISP), and -5.0 (CT+MIX) kcal mol-1. The second phase of the work involved the construction of a CCSD global surface from 208 and 228 energy points for linear and bent conformations, respectively, these being fit to rms errors of only 3.9 and 9.3 cm-1, respectively, below 8000 cm-1. The surface was represented by a flexible analytic form which reproduces the quartic force field at equilibrium, exhibits the proper asymptotic properties, and is generally applicable to ion-molecule systems. The final phase of the study entailed the determination of converged J=0 and J=1 variational eigenstates of the [FHCl]- surface to near the HF+Cl- dissociation threshold by employing Jacobi coordinates and vibrational configuration interaction expansions in terms of natural modals.The fundamental... [ABSTRACT FROM AUTHOR]

Equilibriummolecular structures accurate to 0.001 Å and 0.2° have beendetermined for cis,cis- and trans,trans-1,4-difluorobutadiene by thesemiexperimental mixed estimation method. In this method, structuresare fitted concurrently to equilibrium rotational constants and bondparameters obtained from an intermediate level of electronic structuretheory. The effect of fluorine substitution on the carbon backboneof butadiene is surprisingly small. Definitive energy differencesfor the ground states were computed, employing the focal-point analysis(FPA) technique, between the trans,trans and cis,cis isomers (ΔH°0= 5.6(3) kJ mol–1)and the cis,trans and cis,cis isomers (ΔH°0= 3.2(2) kJ mol–1) of 1,4-difluorobutadiene.These differences confirm the exceptional relationship that the trans,transisomer has the highest energy and the cis,cis isomer the lowest energy,endorsing what was reported earlier on the basis of experimental observationsin benzene solution. [ABSTRACT FROM AUTHOR]

Born−Oppenheimer equilibrium structure (rBOe) estimates are reported for benzene and all 12 possible fluorobenzenes, based on geometry optimizations performed at the coupled cluster level of electronic structure theory including single and double excitations augmented by a perturbational estimate of the effects of connected triple excitations [CCSD(T)] and Gaussian basis sets of at least triple zeta quality. Furthermore, accurate semiexperimental equilibrium (rSEe) structures are determined for C6H6, C6H5F, and 1,2- and 1,3-difluorobenzene. They are obtained through a least-squares structural refinement procedure based on equilibrium rotational constants of as many isotopologues as feasible, determined by correcting experimental vibrationally averaged ground-state rotational constants with computedab initiovibration–rotation interaction constants and electronicg-factors, and using a few structural constraints based on the bestrBOeestimates. TherBOeandrSEeequilibrium structures are in excellent agreement with each other for the four semirigid molecules but in almost all cases they differ significantly from previously determined equilibrium structure estimates based on rotational spectroscopy or gas electron diffraction. The nature of deformations of the benzene ring induced by a single fluorine substitution can be characterized as follows: (a) the strongest effect is the pushing of the ipso carbon atom toward the ring center resulting in a deformation at the ipso [by +2.7(1)°] and ortho [−1.7(1)°] CCC angles, (b) a simultaneous decrease in the ortho CC bond length of the benzene ring by 0.009 Å and (c) a decrease of all the CH bond lengths. Additivity relations concerning the F substitution effects are obtained based on the equilibrium structures of all possible fluorobenzenes. [ABSTRACT FROM AUTHOR]

Abstract: Based on a sample of 38 molecules, 47 accurate equilibrium CO bond lengths have been collected and analyzed. These ultimate experimental , semiexperimental , and Born–Oppenheimer equilibrium structures are compared to estimates from two lower-level techniques of electronic structure theory, MP2(FC)/cc-pVQZ and B3LYP/6-311+G(3df,2pd). A linear relationship is found between the best equilibrium bond lengths and their MP2 or B3LYP estimates. These (and similar) linear relationships permit to estimate the CO bond length with an accuracy of 0.002Å within the full range of 1.10–1.43Å, corresponding to single, double, and triple CO bonds, for a large number of molecules. The variation of the CO bond length is qualitatively explained using the Atoms in Molecules method. In particular, a nice correlation is found between the CO bond length and the bond critical point density and it appears that the CO bond is at the same time covalent and ionic. Conditions which permit the computation of an accurate ab initio Born–Oppenheimer equilibrium structure are discussed. In particular, the core–core and core–valence correlation is investigated and it is shown to roughly increase with the bond length. [Copyright &y& Elsevier]

The principal purpose of this investigation is the determination of the tunneling half-lives of the trans-HCSH → H2CS and the trans-HCSeH → H2CSe unimolecular isomerization reactions at temperatures close to 0 K. To aid these determinations, accurate electronic structure computations were performed, with electron correlation treatments as extensive as CCSDT(Q) and basis sets as large as aug-cc-pCV5Z, for the isomers of [H,H,C,S] and [H,H,C,Se] on their lowest singlet surfaces and for the appropriate transition states yielding structural data for key stationary points characterizing the isomerization reactions. The computational results were subjected to a focal-point analysis (FPA) that yields accurate relative energies with uncertainty estimates. The tunneling half-lives were determined by a simple Eckart-barrier approach and via the more sophisticated though still one-dimensional Wentzel-Kramers- Brillouin (WKB) approximation. Only stationary-point information is needed for the former while an intrinsic reaction path (IRP) is necessary for the latter approach. Both protocols suggest that, unlike for the parent hydroxymethylene (HCOH), at the low temperatures of matrix isolation experiments no tunneling will be observable for the trans-HCSH and trans-HCSeH systems. [ABSTRACT FROM AUTHOR]

Abstract: A new, accurate, global, mass-independent, first-principles potential energy surface (PES) is presented for the ground electronic state of the water molecule. The PES is based on 2200 energy points computed at the all-electron aug-cc-pCV6Z IC-MRCI(8,2) level of electronic structure theory and includes the relativistic one-electron mass-velocity and Darwin corrections. For H2 16O, the PES has a dissociation energy of D 0 = 41109cm−1 and supports 1150 vibrational energy levels up to 41083cm−1. The deviation between the computed and the experimentally measured energy levels is below 15cm−1 for all the states with energies less than 39000cm−1. Characterization of approximate vibrational quantum numbers is performed using several techniques: energy decomposition, wave function plots, normal mode distribution, expectation values of the squares of internal coordinates, and perturbing the bending part of the PES. Vibrational normal mode labels, though often not physically meaningful, have been assigned to all the states below 26500cm−1 and to many more above it, including some highly excited stretching states all the way to dissociation. Issues to do with calculating vibrational band intensities for the higher-lying states are discussed. [Copyright &y& Elsevier]

The proton affinity and the enthalpy of formation of the prototypical carbonyl, formaldehyde, have been determined by the first-principles composite focal-point analysis (FPA) approach. The electronic structure computations employed the all-electron coupled-cluster method with up to single, double, triple, quadruple, and even pentuple excitations. In these computations the aug-cc-p(C)VXZ [X = 2(D), 3(T), 4(Q), 5, and 6] correlation-consistent Gaussian basis sets for C and O were used in conjunction with the corresponding aug-cc-pVXZ (X = 2–6) sets for H. The basis set limit values have been confirmed via explicitly correlated computations. Our FPA study supersedes previous computational work for the proton affinity and to some extent the enthalpy of formation of formaldehyde by accounting for (a) electron correlation beyond the “gold standard” CCSD(T) level; (b) the non-additivity of core electron correlation effects; (c) scalar relativity; (d) diagonal Born–Oppenheimer corrections computed at a correlated level; (e) anharmonicity of zero-point vibrational energies, based on global potential energy surfaces and variational vibrational computations; and (f) thermal corrections to enthalpies by direct summation over rovibrational energy levels. Our final proton affinities at 298.15 (0.0) K are ΔpaHo (H2CO) = 711.02 (704.98) ± 0.39 kJ mol-1. Our final enthalpies of formation at 298.15 (0.0) K are ΔfHo (H2CO) = -109.23 (-105.42) ± 0.33 kJ mol-1. The latter values are based on the enthalpy of the H2 + CO → H2CO reaction but supported by two further reaction schemes, H2O + C → H2CO and 2H + C + O → H2CO. These values, especially ΔpaHo (H2CO), have better accuracy and considerably lower uncertainty than the best previous recommendations and thus should be employed in future studies. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Following an extensive search on the potential energy surfaces (PES) of the natural amino acid L-threonine (Thr) and its allotropic form L-allo-threonine (aThr), 56 and 61 conformers of Thr and aThr, respectively, have been located with the help of density functional theory (DFT). Accurate structures, relative energies, rotational as well as quartic and sextic centrifugal distortion constants, dipole moments, 14N nuclear quadrupole coupling constants, anharmonic vibrational frequencies and double-harmonic infrared intensities have been determined from ab initio electronic structure calculations for the five most stable Thr and aThr conformers. The global minimum, Thr-I, has a cyclic triple H-bond motif with strong OH ··· N, C=O ··· HO, and a weaker NH ··· OH H-bond, where the latter two involves the side chain OH, and an energetically unfavourable trans-COOH arrangement. The best relative energies of the conformers, accurate within ±1 kJ mol-1, have been determined through the first-principles composite focal-point analysis (FPA) approach. There are four and three conformers of Thr and aThr, respectively, within a relative energy of 5 kJ mol-1. Similarly to other amino acids investigated, lower levels of electronic structure theory, especially the Hartree-Fock level, are unable to determine the correct relative energies of the conformers. The rotational, the quartic and sextic centrifugal distortion, and the 14N nuclear quadrupole coupling constants as well as the anharmonic vibrational fundamentals and double-harmonic infrared intensities, all determined using DFT, should aid identification and characterization of the conformers of threonine and allo-threonine by rotational and vibrational spectroscopies, respectively. [ABSTRACT FROM AUTHOR]

We estimated the post-Born-Oppenheimer (post-BO) contribution to electric dipole moments by finite-field derivatives of the diagonal Born-Oppenheimer correction computed with correlated electronic wave functions. The new method is used to examine the effect of isotopic substitution on the dipole moments of the HD, LiH, LiD, and H216O molecules. The non-zero dipole moment of HD is solely due to the post-BO effect and is predicted within a few percent of the best experimental and theoretical results. The post-BO contribution to the dipole moment in LiH and LiD is comparable in magnitude to that in HD, but the difference in total adiabatic dipole moments of LiH and LiD is dominated by the vibrationally averaged BO contribution, and the post-BO contribution is relatively unimportant. However, the post-BO contribution to the dipole moment in H2O is much larger than the vibrationally averaged BO contribution determined by Lodi et al. [J. Chem. Phys. 128, 044304 (2008)] and is twice as large as the discrepancy between their best theoretical BO estimate and the most recent experimental result. Our findings suggest that for species that are well behaved in the BO sense, the post-BO contribution to molecular electric dipole moments can be described within the adiabatic approximation to a few percent accuracy. [ABSTRACT FROM AUTHOR]

Determines the enthalpy of formation of [sup2]πCH at converged levels of electronic structure theory, including high order coupled cluster and full configuration interaction standards. Methods used to determine reference electronic wavefunctions; Process for improvising the enthalpy of formation of the compound; Functions of the scaled higher order correlation procedure used in the experiment.

• Measured Active Rotational-Vibrational Energy Levels technique applied to transitions involving 4 electronic states of NH. • 3002 transitions identified from 18 sources. • 1058 rovibronic energy levels determined. Rovibronic energy levels are determined for four low-lying electronic states (X 3 Σ - , A 3Π, a 1 Δ , and c 1 Π) of the imidogen free radical ( 14 NH) using the M arvel (Measured Active Rotational-Vibrational Energy Levels) technique. Compilation of transitions from both laboratory measurements and solar spectra, found in 18 publications, yields a dataset of 3002 rovibronic transitions forming elements of a measured spectroscopic network (SN). At the end of the MARVEL procedure, the majority of the transitions form a single, self-consistent SN component of 2954 rovibronic transitions and 1058 energy levels, 542, 403, and 58 for the X 3 Σ - , A 3Π, and c 1 Π electronic states, respectively. The a 1 Δ electronic state is characterized by 55 Λ -doublet levels, counting each level only once. Electronic structure computations show that unusually the CCSD(T) method does not accurately predict the a 1 Δ excitation energy even at the complete basis set limit. [ABSTRACT FROM AUTHOR]

Abstract: High-level ab initio electronic structure and variational nuclear motion computations are combined to simulate the spectrum of the water molecule at and above its first dissociation limit. Results of these computations are compared with the related state-selective multi-photon measurements of Grechko et al. [J. Chem. Phys. 138 (2010) 081 103]. Both measured and computed spectra show pronounced structures due to quasi-bound (resonance) states. Traditional resonance features associated with trapping of vibrational or rotational energy of the system are identified and assigned. A strong and broad feature observed slightly above dissociation is found to be associated with direct photodissociation into the continuum. [Copyright &y& Elsevier]