Comparing the accuracy of perturbative and variational calculations for predicting fundamental vibrational frequencies of dihalomethanes.

Three dihalogenated methane derivatives (CHF, CHFCl, and CHCl) 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.

Anharmonic vibrational analysis of s-trans and s-cis conformers of acryloyl fluoride using numerical-analytic Van Vleck operator perturbation theory.

A new gas-phase infrared (IR) spectrum of acryloyl fluoride (ACRF, CHCHCFO) with a resolution of 0.1cm in the range 4000-450cm 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 Møller-Plesset electronic perturbation theory with a correlation consistent Dunning triple-ζ basis set.

Anharmonic Vibrational Analysis of the Infrared and Raman Gas-Phase Spectra of s-trans- and s-gauche-1,3-Butadiene.

A quantum-mechanical (hybrid MP2/cc-pVTZ and CCSD(T)/cc-pVTZ) full quartic potential energy surface (PES) in rectilinear normal coordinates and the second-order operator canonical Van Vleck perturbation theory (CVPT2) are employed to predict the anharmonic vibrational spectra of s-trans- and s-gauche-butadiene (BDE). These predictions are used to interpret their infrared and Raman scattering spectra. New high-temperature Raman spectra in the gas phase are presented in support of assignments for the gauche conformer.

Electron delocalization in polyenes: a semiexperimental equilibrium structure for (3E)-1,3,5-hexatriene and theoretical structures for (3Z,5Z)-, (3E,5E)-, and (3E,5Z)-1,3,5,7-octatetraene.

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.

Accurate equilibrium structures for piperidine and cyclohexane.

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.

Weak acidity of vinyl CH bonds enhanced by halogen substitution.

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.

Semiexperimental equilibrium structures for cis,cis- and trans,trans-1,4-difluorobutadiene by the mixed estimation method and definitive relative energies of the isomers.

Equilibrium molecular structures accurate to 0.001 Å and 0.2° have been determined for cis,cis- and trans,trans-1,4-difluorobutadiene by the semiexperimental mixed estimation method.

Anharmonic vibrational analysis of the gas-phase infrared spectrum of 1,1-difluoroethylene using the operator van Vleck canonical perturbation theory.

Anharmonic vibration frequencies of 1,1-difluoroethylene (11DFE) in the gas phase are predicted by means of the numerical-analytic operator version of the canonical van Vleck perturbation theory in the second and fourth orders (CVPT2 and CVPT4). The full quartic and "semi-diagonal" sextic rectilinear normal coordinate potential energy surfaces, needed for CVPT2 and CVPT4, respectively, were obtained with the MP2/cc-pVTZ quantum-mechanical model. CVPT2 is superior to the traditional second-order vibrational perturbation theory approach (VPT2) because of the uniform general treatment of the Fermi and second-order Darling-Dennison resonances.

Microwave spectra of the deuterium isotopologues of cis-hexatriene and a semiexperimental equilibrium structure.

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.

Semiexperimental equilibrium structure of the lower energy conformer of glycidol by the mixed estimation method.

Rotational constants were determined for (18)O-substituted isotopologues of the lower energy conformer of glycidol, which has an intramolecular inner hydrogen bond from the hydroxyl group to the oxirane ring oxygen. Rotational constants were previously determined for the (13)C and the OD species. These rotational constants have been corrected with the rovibrational constants calculated from an ab initio cubic force field.

Semiexperimental equilibrium structures for the equatorial conformers of N-methylpiperidone and tropinone by the mixed estimation method.

N-Methylpiperidone (MPIP) and tropinone, which contain a structural motif found in numerous alkaloids, are too large to determine an accurate equilibrium structure either by ab initio methods or by experiment. However, the ground state rotational constants of the parent species and of all isotopologues with a substituted heavy atom ((13)C, (15)N, (18)O) are known from microwave spectroscopy. These constants have been corrected for the rovibrational contribution calculated from an ab initio cubic force field.

Analysis of the rotational structure in the high-resolution infrared spectrum of trans-hexatriene-1-13C1: a semiexperimental equilibrium structure for the C6 backbone of trans-hexatriene.

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.

Gas-phase Raman spectra of s-trans- and s-gauche-1,3-butadiene and their deuterated isotopologues.

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 using intense (6 W) green laser excitation and sensitive CCD detection. Hundreds of bands have been observed and assigned for each isotopologue. These spectra provide the best data to date for the s-trans conformer and also provide the first direct observation of the gas-phase Raman bands of the s-gauche conformer.

Semiexperimental equilibrium structure for cis,trans-1,4-difluorobutadiene by the mixed estimation method.

The available experimental rotational constants of cis,trans-1,4-difluorobutadiene do not permit a determination of a complete structure. However, this problem, rather frequent in finding structures, may be solved by the mixed estimation method. The experimental ground state rotational constants are corrected for the rovibrational contribution calculated from an ab initio force field.

Ab initio coupled cluster determination of the equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene.

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 spectra of CF(2)=CHD and CF(2)=CD(2): scaled quantum-chemical force fields and an equilibrium structure for 1,1-difluoroethylene.

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.

Infrared spectra of (12)CF(2)=(12)CH(2) and (12)CF(2)=(13)CH(2), quantum-chemical calculations of anharmonicity, and analyses of resonances.

Infrared spectra obtained in gas and liquid argon phases are reported for (12)CF(2) horizontal line(12)CH(2) and (12)CF(2) horizontal line(13)CH(2). These spectra firmly establish the positions of nu(3)(A(1)) and nu(6)(A(2)) for both isotopomers. Using anharmonicity constants from MP2 calculations, Fermi resonances affecting nu(1)(A(1)), nu(2)(A(1)), nu(3), and nu(8)(B(1)) are analyzed.

High level ab initio energies and structures for the rotamers of 1,3-butadiene.

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.

Semiexperimental equilibrium structure for the C6 backbone of cis-1,3,5-hexatriene; structural evidence for greater pi-electron delocalization with increasing chain length in polyenes.

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.

An investigation of the rotamers of butadiene by high-resolution infrared spectroscopy.

A wide-ranging investigation of high-resolution infrared spectra of 1,3-butadiene was undertaken with the prime objective of finding subbands in the rotational structure attributable to the gauche rotamer, from which information about the molecular structure could be derived. A weak band near 750 cm(-1), which had previously been assigned to the gauche rotamer, has been shown to be a difference band for the trans rotamer. In support of the analysis of this band, the rotational structure, including two hot bands, was analyzed in the C-type band for the nu(12)(a(u)) mode near 525 cm(-1) and in the C-type band near 162 cm(-1) for the nu(13)(a(u)) mode.

Vibrational anharmonicity and harmonic force fields for dichloromethane from quantum-chemical calculations.

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.

Scaled quantum chemical force fields for 1,1-difluorocyclopropane and the influence of vibrational anharmonicity.

Potential functions and harmonic (omega(i)) and anharmonic (nu(i)) fundamental frequencies have been calculated for 1,1-difluorocyclopropane (DFCP) and its d4 and d2 isotopomers using the program Gaussian 03. B3LYP and MP2 models were employed, each with the bases 6-311++G** and cc-pVTZ. Anharmonicity corrections Delta(i) = omega(i) - nu(i) are listed and shown to be different for symmetric and antisymmetric CH stretching modes in situations where Fermi resonance appears to be absent.

Ab initio structures for 90 degrees -twisted s-trans-1,3-butadiene and cyclooctatetraene: the naked sp2-sp2 bond.

The bond length of a carbon-carbon sp2-sp2 sigma-bond without the perturbing effects of pi-interactions has been estimated by high level calculations on two prototypical systems: a 90 degrees -twisted form of butadiene and the tub conformer of cyclooctatetraene. The former system yields a value of 1.4818 A, considerably longer than previous estimates.

Vibrational spectroscopy of 1,1-difluorocyclopropane-d0, -d2, and -d4: the equilibrium structure of difluorocyclopropane.

IR and Raman spectra are reported for 1,1-difluorocyclopropane-d0, -d2, and -d4, and complete assignments of vibrational fundamentals are given for these species. These assignments are consistent with predictions of frequencies, intensities, and Raman depolarization ratios computed with the B3LYP/cc-pVTZ quantum chemical (QC) model. Ground state rotational constants for five 13C and deuterium isotopomers, obtained from published microwave spectra, were "corrected" into equilibrium rotational constants.