Rotational isomemsm in difluoroacetyl fluoride: Part II. AB initio calculations

The potential energy function for internal rotation of difluoroacetylfluoride around the C-C axis has been obtained by ab initio SCF calculations in a Gaussian basis set. Two minima are predicted with H and F atoms trans (α = O²) and gauche (α = 107–108°), respectively. Gauche- DFAF is incorrectly predicted to have the lower energy, but the addition or bond functions to the basis reduces the gauche-trans energy difference to ?0.1 kcal mole^?1. Dipole moments and torsional excitation energies are reported for both conformations and the significance of the computed potential function is critically analyzed. No support has been found for the suggestion that the C-F bonds in the CHF₂-group of gauche-DFAF are significantly different.     

Vibrational studies,barrier height and thermodynamic functions for biomolecules: 5-Trifluoromethyl uracil

Laser Raman (50–4000 cm⁻¹) and IR (200–4000 cm⁻¹) spectra of 5-trifluoromethyl uracil have been recorded and analysed. It has been possible to assign all the 39 (26a′+13a″) normal modes of vibration. Consistent assignments have been made for the internal modes of the CF₃ group, especially for the antisymmetric CF₃ stretching and bending modes. Using thus assigned vibrational frequencies and assumed structural parameters, thermodynamic functions, in the temperature range 100–1000 K, have been computed and the barrier to the internal rotation for the CF₃ top has been determined.     

Internal rotation in mononitro-n-alkyl radicals

Internal rotation in the C·H₂(CH₂) _n NO₂ (n ≤ 7) type radicals has been studied. 44 potential functions of the internal rotation, V(φ), have been calculated taking advantage of the B3LYP/6-311++(3df,3pd) and MP2/6-311++(3df,3pd) methods. The trends observed in the series of parameters characterizing the internal rotation have been explained in view of the electron clouds conjugation, the inductive effect of the end groups, the gauche effect, and the rotation tops interaction. The coefficients of V(φ) have been shown to depend predominantly on the nearest surrounding of the rotation axis. Based on this, the generalized functions, V _av(φ), have been developed, their coefficients being dependent exclusively on the rotating bond position. Such functions are convenient for molecular modeling applications.     

Infrared,Raman and NMR spectra,conformational stability,and ab initio calculations of divinylmethoxyborane

The infrared spectra (4000–50 cm⁻¹) of gaseous and solid divinylmethoxyborane, (CH₂=CH)₂BOCH₃, as well as the Raman spectra (3500–20 cm⁻¹) of the liquid and solid have been recorded. Qualitative depolarization values have been obtained from the Raman spectrum of the liquid. All normal modes, except the torsions, have been assigned based on infrared band contours, depolarization values, group frequencies, and normal coordinate calculations. From a comparison of the spectra in the fluid and solid states, it is concluded that the molecule exists predominantly in a single conformation in all physical states. Frequencies and potential energy distributions for the normal modes have been calculated with the 3–21G basis set. A comparison of these calculated frequencies to the observed spectra is consistent with the predominant form having a “planar” heavy atom skeleton with C_s, symmetry. From the variable low temperature ¹³C NMR data, a barrier to rotation about the B-O bond of 10.1 ± 0.1 kcal mol⁻¹ has been determined, which is in excellent agreement with a barrier of 8.5 kcal mol"1 obtained from ab initio calculations. Structural parameters, conformational stability, and barriers to internal rotation have been obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21G and 6–31G^* basis sets. The results are compared to the corresponding data for some similar organoboranes.     

Far-infrared spectrum,barrier to internal rotation,ro structure,and ab initio calculations for acetylacetylene

The far-infrared spectrum has been recorded from 50 to 360 cm^–1 at a resolution of 0.10 cm^–1 for acetyiacetylene (1-butyne-3-one], CH₃C(O)CCH. The fundamental methyl torsion has been observed at 117.94 cm^–1, from which a periodic barrier to internal rotation has been calculated to be 346 cm^–1 (989 cal mol^–1]. Infrared spectra (3500-50 cm^–1] of the gas and solid and the Raman spectra (3500-100 cm^–1) of the gas, liquid, and solid are reported. Utilizing previously reported rotational constants for three isotopic species,r _o structural parameters have been determined for the heavy-atom skeleton. The fundamental vibrational frequencies, barrier to internal rotation, and structural parameters that have been obtained experimentally are compared to those obtained from ab initio Hartree-Fock calculations employing 3-21G, 6-31G, and DZ basis sets and to the corresponding quantities for some similar molecules.     

Ab initio investigation of tautomeric stability,molecular structure,and internal rotation of methylphosphonic dicyanide,methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)₂, McOP(CN)₂, and their isocyano analogs, MeP(O)(NC)₂ and MeOP(NC)₂, have been calculated by theab initio SCF method and with inclusion of electron correlation effects according to the second-order Muuller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25–30 kcal mol^–1 lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15–25 kcal mol^–1 more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for thetrans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to thecis-arrangement. The equilibrium oxo-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.Translated from izvestiyaAkademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1104–1115, May, 1996.     

Spectra and structure of small ring compounds. LXII. Conformational stability,structural parameters,ab initio calculations and vibrational assignment of cyclopropylmethyl ketone

The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies of cyclopropylmethyl ketone, c-C₃H₅C(CH₃)O, have been obtained from Hartree—Fock ab initio calculations with the RHF/3-21G and RHF/6-31G* basis sets, as well as the 6-31G* basis set with electron correlation at the MP2 level, and the results are compared to those obtained from experiment. The data are consistent with the predominant rotamer having the cis conformation (carbonyl bond cis to the ring). A second form, having a “near” trans structure, is calculated to have a larger total dipole moment than the cis form, which accounts for its increased abundance in the liquid compared to that in the gas. A complete vibrational assignment is proposed based on experimental data and normal coordinate results from the ab initio calculations. The asymmetric torsional barrier has been calculated to be approximately 2000 cm⁻¹ and this result along with others is compared to the corresponding data obtained from both experiment and theory for the cyclopropylcarbonyl halides.     

Conformational stability of peroxynitric acid molecule and the barriers to internal rotation about the O−O and N−O bonds

The structure of conformers and potential curves of the internal rotation (PCR) about the O?O and N?O bonds in peroxynitric acid (PNA) were calculated by the unrestricted Hartree-Fock-Roothaan method. The standard valence-split 6–31G and 6–31G^* basis sets were used. The presence of two maxima on every curve has been shown. To refine the values of barriers to the internal rotation in the regions of minima and maxima of PCR, calculations taking into account the electron correlation energy have been carried out at the second- and fourth-order Møller-Plesset level of perturbation theory (MP2 and MP4, respectively). At the MP4/6-61G^* level of approximation, the barriers to the rotation about the O?O bond are equal to 8.6 kJ mol^?1 and 14.7 kJ mol^?1, and both barriers to the rotation about the N?O bond are equal to 33.5 kJ mol^?1. The results are compared with those published for PCR in hydrogen peroxide and peroxynitric acid.     

Composite spectral density functions for the interpretation of the 13C NMR relaxation behavior of polymers containing hydrocarbon side chains. Free and restricted side chain motion

¹³C NMR NT₁ and NOE have been calculated by using composite spectral density functions describing polymer chain segmental motion and internal rotation of a hydrocarbon side chain attached to the polymer backbone. Numerical results at two magnetic fields are presented as a function of the various motional parameters characterizing the various models. NT₁ and NOE relaxation parameters are well behaved and appear to have practical value for describing the dynamics of these systems. The models have been applied to the relaxation data of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) in toluene solutions. The dynamics of the two polymers are characterized by a very localized backbone motion and restricted internal rotation about successive C? C bonds of the side chains. © 1995 John Wiley & Sons, Inc.     

Effect of free valence on the characteristics of internal rotation in <Emphasis Type="Italic">n</Emphasis>-alkyl radicals

A study of internal rotation in the radicals n-C _n H_2n+1C^·H₂, (2 ≤ n ≤ 7) was carried out for the case of rotation around the bonds not including the radical center. 21 potential functions of internal rotation V(φ) were calculated. The coefficients in V(φ) were shown to depend only on the immediate environment at the bond of rotation. Characteristics of internal rotation in n-alkyl radicals were compared with related parameters of the corresponding alkane molecules. The generalized function V _av(φ) with the coefficients defined only by the position of the bond of rotation in the hydrocarbon chain and possessing the transposition property were proposed. The functions V _av(φ) were recommended for the simulation of the structure and properties of large molecules containing hydrocarbon fragments. This work continues a systematic study on the characteristics of the internal rotation in the n-alkane molecules and the groups containing free valence in n-alkyl radicals.     

Computational and dynamic NMR investigation of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane

The structure and rotational barrier for the mesityl-silicon bond of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane have been investigated by ¹H- and ¹³C-variable temperature nuclear magnetic resonance (NMR) as well as by density functional theory structural calculations. The calculations show that the lowest energy structure has C₂ symmetry with nonequivalent ortho methyl groups, consistent with the crystal structure and solution NMR. The nonequivalent ortho methyl groups exchange through a C_s transition state with a calculated relative free energy of 11.0 kcal mol⁻¹. The barrier for this rotation found by dynamic NMR is 13.4 ± 0.2 kcal mol⁻¹ at 298 K.     

Infrared and raman spectra,vibrational assignment and barriers to internal rotation for methyldisilylamine

The Raman spectra of gaseous and liquid (SiH₃)₂NCH₃ and (SiH₃)₂NCD₃ have been recorded to within 10 cm^?1 of the exciting line. The IR spectra of (SiH₃)₂NCH₃ and (SiH₃)₂NCD₃ have been recorded from 80 cm^?1 to 3800 cm^?1 in the gaseous state, and from 80 cm^?1 to 450 cm^?1 in the solid state. A vibrational assignment has been made, and from the low-frequency vibrational data, an upper limit of 3.3 kcal mol^?1 was calculated for the barrier to internal rotation of the silyi groups, whereas a barrier of ~450 cal was calculated for internal rotation of the methyl group. It is concluded that there exists a significantly strong d_π—p_π interaction in methyldisilylamine.     

Molecular structure and conformational transitions of dichloroacetylchloride

RHF and MP2 techniques in 6–31G(d) basis set have been used to determine the structure of the isolated molecule CHCl₂COCl in two stable conformations (cis-and gosh-), as well as in transition states arising due to the rotary motion of CHCl₂ group around the C—C bond. The energy gap between the conformers and the relevant potential barriers has been calculated using the obtained potential dependence of the internal rotation. Plausible conformation of dichloroacetylchloride is discussed on the basis of ³⁵Cl NQR.     

Dynamic structure of organic compounds in solution according to NMR data and quantum mechanical calculations: I. Soman

Dynamic structure of Soman diastereoisomers has been studied with the goal of obtaining accurate information to simulate molecular mechanisms of its action on living systems. The potential energy surface for internal rotation about the single P–O and O–C bonds has been constructed in terms of the Møller–Plesset second-order perturbation theory using 6-311G(d,p) basis set. The relative contributions of different conformers have been estimated by solving the vibrational problem according to the large-amplitude vibration model. The conformational dependences of the ⁴J_CF and ³J_CP coupling constants for the S,S and S,R diastereoisomers of Soman have been calculated at the FPT DFT B3LYP/6-311++G(2df,2p) level of theory. The calculated vibrationally averaged coupling constants have been compared with the available experimental data to determine the structure of the most toxic Soman stereoisomer.     

Torsion potentials and electronic structure of trifluoromethoxy- and trifluoromethylthiobenzene: an ab initio study

Potential functions of internal rotation about the C_sp²X bonds in molecules C₆H₅XCF₃ (X=O, S) were calculated at the second-order Møller-Plesset perturbation level of theory with 6-31G(d) basis set. The profile of the potential function and the rotation barrier (ΔE^#=3.0 kJ/mol) found for C₆H₅OCF₃ suggest that, depending on experimental conditions, there can be either free rotation about the C_sp²O bond or the conformational equilibrium is shifted to the side of the orthogonal form. The rotational barrier for C₆H₅SCF₃ is 14.7 kJ/mol and the molecule exists in the stable orthogonal conformation. The nature of hybridization, energy and population of lone electron pairs (LPs) on the oxygen and sulfur atoms were considered by using the Natural Bond Orbital (NBO) method. The energy of interactions of the LPs with antibonding π^∗-orbitals of the aromatic moiety were estimated for different conformations. The distribution of electron density in the molecules was discussed. The results were compared with analogous calculations on the molecules C₆H₅XCH₃.     

Vibrational spectra,methyl torsional potential functions,internal rotational potential functions,and conformational properties of ethyl vinyl ether

The vibrational spectra of ethyl vinyl ether in both the fluid and solid states have been recorded from 20 to 3500 cm^?1. The 33 fundamental modes of vibration have been assigned. Three rotational isomers have been observed and their structures have been determined. The most stable conformer, s-cis/s-trans, is planar and of C_s symmetry. The two less stable rotamers, skew/s-trans and skew/gauche, are non-planar and of C_i molecular symmetry. The barrier to internal rotation of the methyl rotor has been determined for each conformation; these barriers are 3.43 kcal mol^?1 (s-cis/s-trans), 3.35 kcal mol^?1 (skew/s-trans) and 3.19 kcal mol^?1 (skew/gauche). A potential function for each of the two asymmetric internal rotations has been calculated and barriers to conformer interconversion have been determined. From the asymmetric potential function calculations, ΔH, the enthalpy difference between the conformers, has been determined. The s-cis/s-trans conformer is 1.87 kcal mol^?1 more stable than the skew/s-trans conformer; the skew/s-trans conformer is more stable than the skew/gauche conformer by 1.10 kcal mol^?1. The energetics of conformer interconversion and methyl internal rotation have been described in terms of molecular geometry and non-bonded interactions. These results are compared to those found in other alkyl vinyl and dialkyl ethers.     

Ab initio calculation of the potential functions for internal rotation around the CS bonds in simple ethene thiols

Ab initio calculations with complete geometry optimisation have been used to study internal rotation in compounds of the type XCH = CHSH, X = CN, H, CH₃ and F, with X located trans to the sulphur atom. Potential functions for the CS torsion have been obtained for each case and it has been established that the dominant framework changes accompanying internal rotation in these molecules involve the CCS angle and CS bond length. Furthermore, it has been shown that the nature of the substituent X significantly affects the molecular conformation of the SH group. The observed trends are discussed in terms of a simple model.     

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

Rotational spectra of ortho-nitrotoluene (2-NT) and para-nitrotoluene (4-NT) have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation splittings arising from the methyl group. For 2-NT, an anisotropic internal rotation of coupled −CH₃ and −NO₂ torsional motions was identified by quantum chemistry calculations and discussed from the results of the MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier.     

Nonempirical quantum chemical calculation for nitramide,its chloro- and methyl-substituted derivatives. II. Structures and energies of transition states for internal rotation and inversion of the amino group

For five N-nitramines (H₂NNO₂, MeNHNO₂, ClNHNO₂, MeNClNO₂, Me₂NNO₂) using the program GAUSSIAN-90 we have carried out quantum chemical calculations by the restricted Hartree—Fock method, taking into account electron correlation by second-order Møller—Plesset perturbation theory in a standard 6–31G^* basis. In this paper, we consider the transition states for inversion of the amine nitrogen atom and rotation about the NN bond. We have obtained data on the changes in the geometric parameters during inversion and rotation. The changes in the NN bond length are especially significant they increase by 0.06–0.08 Å in the transition states for internal rotation compared with the equilibrium forms. We have calculated the barriers to inversion and internal rotation, the height of which strongly depends on the electronegativity of the substituents on the amine nitrogen atom. Estimates of the barriers to inversion lie within the range 0.4–6.0 kcal/mole while estimates of the barriers to rotation lie within the range 6–13 kcal/mole, which are 1.5–2 times lower than in amides and N-nitrosoamines.Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 12–19, January–February, 1993.