Determination of force fields for formaldehyde,acetaldehyde and acetone by the CNDO/force method

CNDO/Force calculations have been done for formaldehyde, acetaldehyde and acetone, and the theoretical force fields evaluated. Experimental force fields are obtained from vibrational frequencies using the least-squares refinement method. The initial force fields considered are based on the bending and interaction force constants obtained from the CNDO/Force calculations and the stretching force constants transferred from chemically related molecules. Vibrational frequencies of H₂CO, D₂CO, HDCO, H₂¹³CO and D₂¹³CO for formaldehyde, CH₃CHO, CH₃CDO, CD₃CHO, CD₃CDO and CH₂DCHO for acetaldehyde, and CH₃COCH₃ CD₃COCH₃ and CD₃COCD₃ for acetone are employed in the force field refinements. The final force fields obtained are found to be reasonable with respect to the diagonal and interaction force constants.     

Vibrational spectra and molecular structure of methyl vinyl ether

The IR and Raman spectra of methyl-d₃ vinyl and methyl-d₃ vinyl-d₃ ethers are reported (along with earlier published spectra of the undeuterated molecule). The normal coordinate analysis, based upon CNDO/2 force constant calculations, confirms the absence of significant changes in CH₃ group dynamical properties in methyl vinyl ether compared to saturated ethers. Normal coordinate calculations of possible rotamers (cis structure force field with certain assumptions has been used) favours the planar trans (or close to it) structure of the second isomer.     

Vibrational Spectra, Assignment, Conformational Stability and Ab Initio/DFT Calculations for 1-Nitropropane

The infrared spectra (4000–400 cm^{– 1}) of solid and the Raman spectra (3500–30 cm^{– 1}) of liquid and solid 1-nitropropane, CH₃CH₂CH₂NO₂, have been registered. Both the trans and gauche conformers have been identified in the fluid phase, while the trans form remains in the stable solid. Temperature dependence (190–230K) of the liquid 1-nitropropane Raman spectra has been carried out. From these data, the enthalpy difference was determined to be 870 ± 105 J-mol^–1, with the gauche conformer being the more stable rotamer. Ab initio and DFT calculations at different levels of approximation (HF, MP2, B3LYP, B3PW91) gave optimized geometries, harmonic force fields, and vibrational frequencies for the trans and gauche conformers. All the calculations (except the B3PW91/6-31G* level) predicted gauche as the low-energy conformer. Theoretical force constants are analyzed for formulating constraints in the molecular force field model of 1-nitropropane.     

Photoacoustic study of vibration to translation-rotation transfer inch4 and cd4

The vibrational deactivation of CH₄ and CD₄ is studied experimentally in the gas phase using the photoacoustic method. Excitations of v₄ and v₃ modes are performed. The corresponding kinetic models are established using the fact that vibration-to-vibrati transfers are very efficient and hold the adjacent vibrational levels in a quasi-equilibrium distribution. The models are tested between 140 and 376 K and the vibration to translation-rotation rate constants are determined in pure CH₄ pure CD₄, and CH₄-CD₄ mixtures.     

Conformational Studies of Cyclopropylmethyl Ketone from Temperature-Dependent FT–IR Spectra of Xenon Solutions

Variable temperature (–60 to –100°C) studies of the infrared spectra (3500–400 cm^–1) of cyclopropylmethyl ketone, c-C₃H₅C(CH₃)O, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the cis and near-trans conformers, the enthalpy difference has been determined to be 269 ± 26 cm^–1 (3.22 ± 0.31 kJ/mol) with the cis conformer (oxygen atom cis to the three-membered ring) the more stable rotamer. From these data it is estimated that 79 ± 3% of the cis form is present at ambient temperature. Ab initio calculations have been carried out with different basis sets up to 6-311+G(2df,2pd) at the restricted Hartree–Fock and/or with full electron correlation by the perturbation method to second order (MP2) from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions that the cis form is the more stable conformer. A complete vibrational assignment is given for the cis conformer, which is supported from a normal coordinate calculation utilizing ab initio force constants. Several of the fundamentals of the near-trans conformer have been identified and assigned. Adjusted r ₀ structural parameters have been obtained from combined ab initio predicted values and previously reported rotational constants from the microwave investigation. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

Vibrational spectra and force constants of Ch3HgI and CD3HgI

The infrared and Raman spectra of CH₃HgI and CD₃HgI were studied in the solid state. All the fundamental wavenumbers are assigned. A general harmonic force field was used as the basis, and the force constants were modified by means of the Jacobian matrix. The force constants fit the observed wavenumbers better than 1 %. The normal coordinates are also given.     

The infrared spectra and normal coordinate analysis of syn and anti acetaldoxime

The infrared spectra of mixtures of syn and anti acetaldoxime and its deuterated analogues CH₃CHNOD, CD₃CHNOH, CD₃CDNOH and CH₃CDNOH have been recorded. The syn and anti isomers of CH₃CHNOH, CD₃CHNOH, CD₃CDNOH and CH₃CDNOH have been separated by gas chromatography [1]. The infrared spectra of separated isomers in CS₂ solution have been recorded and the assignment of ten in-plane vibrations made. From a normal coordinate analysis the Urey-Bradley force field, the potential energy distribution and additional information about assignments have been obtained.For the anti isomer the simple Urey-Bradley force field gives satisfactory agreement between the calculated and measured frequencies. For the syn isomer it is necessary to take into account the interactions between atoms separated by three bonds.     

Infrared and Raman spectra and normal coordinate calculations for methylisothiocyanate

The infrared spectra (3200-50 cm^?1) of gaseous and solid CH₃NCS and CD₃NCS and the Raman spectra (3200-10 cm^?1) of the liquids and solids have been recorded. The spectra have been interpreted on the basis of a “pseudo-symmetric top” with C_3v symmetry. An assignment of the fundamental vibrations in both molecules, based on their infrared band contours, depolarization values and group frequencies, is given and discussed. Particularly interesting is the low-frequency region where band maxima were observed at 152 and 80 cm^?1 for CH₃NCS and 139 and 71 cm^?1 for CD₃NCS in the infrared spectra of the gases. A normal coordinate analysis has also been carried out based on C_3v symmetry. Considerable mixing was found between the C_αN stretch and NCS symmetric stretch in both isotopic species. The other normal modes in CH₃NCS are reasonably pure but, for the CD₃NCS molecule, considerable mixing was found between the CD₃ stretches and NCS antisymmetric stretch. The proposed vibrational assignment and the results of the normal coordinate calculations are discussed and compared with the results obtained for similar molecules.     

Schwingungsverhalten des syn- und anti-acetaldoxims: 2. Mitt. normalkoordinatenbehandlung

Normal coordinate calculations on CH₃CHNOH, CH₃CHNOD, CH₃CDNOD and CD₃CDNOD have been carried out applying the general harmonic valence force field. The calculated frequencies of the normal vibrations considered agree satisfactorily with the corresponding values obtained from the spectra. The potential energy distributions have also been calculated; they were used to interpret the behaviour of characteristic vibrations.     

Theoretical study of anharmonic force field and spectroscopic constants for 1-chlorophosphaethene,CH2PCl and CD2PCl

The anharmonic force field and spectroscopy constants for CH₂PCl are determined using CCSD(T), VPT2, and density functional theory employing cc-pVQZ basis sets. The molecule structure, rotational spectroscopic constants, and vibrational wave numbers are compared with the available experimental data. Anharmonicity constants, vibration-rotation interaction constants and cubic force constants are predicted. Vibrational wave numbers and rotational constants for CD₂PCl are also determined using the same levels. The isotopic shifts of vibrational wave numbers are remarkable by D atom substitution for 1-chlorophosphaethene.     

Vibrational spectra and normal coordinate calculations for (CH3)3GECN and (CD3)3GECN

Infrared spectra of (CH₃)₃GeCN and (CD₃)₃GeCN have been obtained over the range of 4000–100 cm^?1 from solutions of benzene and dichloromethane, and solid films deposited on windows at liquid nitrogen temperature. Raman spectra have been photographed in dichloromethane solution. Assignments for all fundamentals except the internal torsions have been made, and normal coordinate calculations carried out by a symmetry force field to confirm the proposed assignments.     

All-trans- and t,T,t,C,t,T,t-deca-1,3,5,7,9-Pentaenes: Ab Initio Structures,Vibrational Analyses,and Some Regularities in the Series of Related Molecules

Total geometry optimization and calculation of the force constants for all-transand t,T,t,C,t,T,tdeca-1,3,5,7,9-pentaene were carried out at the ab initio, HF/6-31G level. The HF/6-31G//HF/ 6-31G force fields were modified using empirical scale factors transferred from trans-buta-1,3-diene augmented by an additional scale factor for the central formal carbon-carbon double bond coordinates (determined previously for all-trans-hexa-1,3,5-triene). The total number of scale factors was seven. The vibrational problems for both decapentaenes were solved using the respective scaled HF/6-31G//HF/6-31G force field. Infrared intensities and Raman activities were calculated from the unscaled HF/6-31G//HF/6-31G force fields. Complete assignment of all the fundamental vibrational frequencies is given. Geometrical parameters, vibrational frequencies and force constants are compared with the corresponding values of buta-1,3-diene, hexa-1,3,5-triene and octa-1,3,5,7-tetraene. Regularities in the properties of this molecular series are discussed. Special attention is given to the possibility of using the vibrational spectra for detection of distortions from the regular trans structure of these oligoenes.     

The vibrational and NMR spectra,conformations and ab initio calculations of aminomethylene,propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H₂N-CH=C(CN)₂], (methylamino)methylene propanedinitrile (MAM) [CH₃NH-CH=C(CN)₂] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH₃)₂N-CH=C(CN)₂] as solids and solutes in various solvents have been recorded in the region 4000-50 cm^–1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference H ⁰ between the conformers was measured to be 3.7±1.4 kJ mol^–1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol^–1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol^–1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor^–1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.     

Matrix and gas infrared spectra of nitroethane isotopic species

IR and FIR gas phase and Ar matrix isolation spectra of the isotopic species of nitroethane, CH₃CH₂NO₂, CH₃CD₂NO₂ and CD₃CD₂NO₂ are reported. An assignment will be proposed, based mainly on matrix spectra and normal coordinate analysis. An approximate 25 parameter valence force field is given. The analysis of the gas phase spectra is complicated by the fact that only very few band contours comparable to quasirigid model envelopes occur. The phenomenon and unusual band widths and splittings in the matrix spectra are related to the nearly free internal rotation of the nitro group. The Q branch sequence associated with the methyl torsion shows a remarkable splitting into doublets probably also related to the NO₂ rotation. A value of 1080 ± 20 cm^?1 for the methyl torsional barrier is derived.     

Gas and matrix infrared spectra of n-methyl-ethylidenimine and valence force field

Infrared spectra of gaseous and matrix isolated trans-CH₃N:CHCH₃, CD₃N:CHCH₃ and CH_N:CDCD₃ have been recorded from 4000 to 200 cm^?1 under medium resolution. Nearly complete assignments have been established based upon rotational contours, isotope shifts and group frequency correlations. A number of Fermi resonances have been detected mainly related to CH(D) stretching fundamentals. ln particular, the light N-methyl group gives rise to a complicated Fermi multiplet in the CH-stretching region. An examination of Q sub-band structures resulted in the identification of a number of Coriolis resonances, which in most cases involve pairs of nearly coinciding ‘E-type“ methyl deformation fundamentals. The rotational fine structure of three bands has been analysed by means of combination differences providing estimates for effective rotational constants A(u)-$\text{1}\text{2}$ [B(u)+C(u)]. From a normal coordinate analy a 30 parameter valence force field has been obtained, which allows an approximation of 85 experimental frequencies with an RMS deviation of 7.4 cm^?1.     

The stable form of 1, 1, 1-trifluoro-3-chloropropane molecules in the crystal II modification

The vibrational spectra of CF₃CH₂CH₂Cl in different states of aggregation is investigated and the normal coordinate analysis of trans- and gauche-conformers is carried out. The assignments in vibrational spectra for both the conformers of CF₃CH₂CH₂Cl are given. It is shown that the stable crystalline modification, crystal II, is formed by the trans-conformer of the molecule.     

Der massenspektrometrische Zerfall isomerer Diacetamido-cyclo-hexane,ihrer N-Phenäthyl-Derivate und Bis (acetamidomethyl)cyclohexane. 32. Mitteilung über massenspektrometrische Untersuchungen,,

The Mass Spectral Decomposition of Isomeric Diacetamido-cyclohexanes, their N-Phenethyl-Derivatives and Bis(acetamidomethyl)cyclohexanes In the mass spectra of the six isomeric diacetamidocyclohexanes 2--4 (cis and trans each, Scheme 2) as well as of the six isomeric bis(acetamidomethyl)cyclohexanes 6--8 (cis and trans each, Scheme 5) are clear differences between the constitutional isomers, whereas cis/trans isomers show very similar spectra. The lack of stereospecific fragmentations is explained by loss of configurational integrity of the molecular ion before fragmentation. However, the mass spectral fragmentation of epimeric diamidocyclohexanes becomes very stereospecific by the introduction of a phenethyl group on one of the nitrogen atoms: this group avoids epimerization of the molecular ion prior to fragmentation. In the N-phenethyl derivatives 10, 11, 13 and 14 (Scheme 8) the typical fragmentations of the cis-isomer after loss of ·C₇H₇ from the molecular ion are the elimination of CH₂CO by formation of cyclic ions, and the loss of p-toluenesulfonic acid or benzoic acid, respectively, with subsequent elimination of CH₃CN (Scheme 9). In the trans-isomer the typical fragmentations are the loss of the side chain bearing a tertiary nitrogen atom, and the elimination of the tosyl or benzoyl radical, respectively, with subsequent loss of CH₃CONH₂ (Scheme 10).     

Vibrational spectra and force field of dimethylphosphines

Vibrational spectra in the range 200–3000 cm^?1 are reported and assigned for the species (CH₃)₂PH, (CH₃)₂PD, (CD₃)₂PH, (CD₃)₂PD, CH₃CD₃PH and CH₃CD₃PD. The spectra in the range 1020–500 cm^?1 are complicated due to the coupling between δPH, ?Me and the skeletal modes of the molecule. Interpretation is only possible through a force field which is markedly different from an earlier one of dimethyl sulphide. This force field predicts uncoupled δPH frequencies of 835 (a) and 909 cm^?1 (a), couples PH bending largely to out-of-skeletal plane methyl rocking (?_i) and includes a low p¦¦(a) bending constant, a high skeletal bending constant and unusual signs for two interaction constants. In the crystalline phase at 78 K, the two methyl groups are non-equivalent.     

Isomerization reactions of RSNO (R=H,C<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>H<Subscript>2<Emphasis Type="Italic">n</Emphasis>+1</Subscript><Emphasis Type="Italic">n</Emphasis>≤ 4)

We have applied various theoretical methods to gain detailed insights into the isomers as well as the transition states (TSs) along the corresponding reaction pathways for RSNO (R=H, C _n H_2n+1 n ≤ 4). On the basis of G2 and G2MP2 results, the relative order of stability for R=H is estimated to be trans-HSNO > cis-HSNO > HNSO > cis-HONS trans-HONS, while it is cis-CH₃SNO trans-CH₃SNO > CH₃NSO > trans-CH₃ONS > cis-CH₃ONS for R=CH₃. A similar trend is also obtained from the B3P86 method with considerably less computing effort if the nearly isoenergetic isomers cis-HONS and trans-HONS are ignored. Based on the results of B3P86, cis-RSNO is more stable than trans-RSNO when R=H is replaced by alkyl groups except for R=t-Bu. Natural bond orbital analyses allow us to explore whether the high reactivity of S-nitrosothiols is due to the strong negative hyperconjugation (). The mesomeric effect of S-nitrosothiols, although non-negligible, does not cause the breakage of N–O bond due to the compensation of columbic attraction between N and O.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

An analysis of the vibrational structure of high-resolution UV spectra and long-wave IR Fourier transform spectra in studies of internal rotation in molecules

The methods for analyzing the vibrational structure of high-resolution UV spectra and long-wave IR Fourier transform spectra in studies of internal rotation in α,β-unsaturated carbonyl compounds R₄R₃C=CR₂-COR₁ (R₁ = F, Cl; R₂ = R₃ = R₄ = H, CH₃) are compared. These methods were found to give different experimental values for systems of torsional vibration energy levels up to high quantum numbers, torsional frequencies (0–1 transitions), and anharmonicity coefficients x ₁₁ for trans and cis isomers of the same molecules in the ground electronic state (S ₀). It was shown that the experimental technique for analyzing the vibrational structure of UV spectra excludes the hydrolysis of compounds under study. Taking into account Fermi resonance and numerous Deslandres tables constructed for trans and cis isomers provides reliable determination of values necessary for the construction of internal rotation potential functions, because they are multiply repeated in various Deslandres tables. An analysis of the vibrational structure of UV spectra gives more reliable V _n internal rotation potential function parameters. The V _n parameter values were substantiated by quantum-mechanical calculations performed by other authors.