Internal Rotation Barrier Research Articles

Potential energy surface and rovibrational calculations for the ${\rm Mg}^+$ Mg +–${\rm H}_2$H2 and ${\rm Mg}^+$ Mg +–${\rm D}_2$D2 complexes

A three-dimensional potential energy surface is developed to describe the structure and dynamical behavior of the Mg(+)-H(2) and Mg(+)-D(2) complexes. Ab initio points calculated using the RCCSD(T) method and aug-cc-pVQZ basis set (augmented by bond functions) are fitted using a reproducing kernel Hilbert space method [Ho and Rabitz, J. Chem. Phys. 104, 2584 (1996)] to generate an analytical representation of the potential energy surface. The calculations confirm that Mg(+)-H(2) and Mg(+)-D(2) essentially consist of a Mg(+) atomic cation attached, respectively, to a moderately perturbed H(2) or D(2) molecule in a T-shaped configuration with an intermolecular separation of 2.62 Å and a well depth of D(e) = 842 cm(-1). The barrier for internal rotation through the linear configuration is 689 cm(-1). Interaction with the Mg(+) ion is predicted to increase the H(2) molecule's bond-length by 0.008 Å. Variational rovibrational energy level calculations using the new potential energy surface predict a dissociation energy of 614 cm(-1) for Mg(+)-H(2) and 716 cm(-1) for Mg(+)-D(2). The H-H and D-D stretch band centers are predicted to occur at 4059.4 and 2929.2 cm(-1), respectively, overestimating measured values by 3.9 and 2.6 cm(-1). For Mg(+)-H(2) and Mg(+)-D(2), the experimental B and C rotational constants exceed the calculated values by ∼1.3%, suggesting that the calculated potential energy surface slightly overestimates the intermolecular separation. An ab initio dipole moment function is used to simulate the infrared spectra of both complexes.

Conformational behaviour, hydrogen bond competition and intramolecular dynamics in vanillin derivatives: acetovanillone and 6-hydroxy-3-methoxyacetophenone

The conformational landscape of the structural isomers acetovanillone (apocynin, AV) and 6-hydroxy-3-methoxyacetophenone (HMAP) has been investigated in a supersonic jet using Fourier transform microwave spectroscopy. Two conformers have been detected in the jet-cooled expansion for each molecule (s-cis and s-trans in AV; s-trans and a-trans for HMAP), differing in the relative orientation of the acetyl and methoxy groups. Both molecules are stabilized by O-H···O or O-H···O=C hydroxyl intramolecular hydrogen bonds, either constraining the local conformations of the methoxy group in AV, or that of the acetyl group in HMAP. Internal rotation splittings have been observed in both conformers of each molecule, originated by the acetyl group, that yield information on the influence of the intramolecular hydrogen bonds on the methyl torsion. The similar internal rotation barriers in both molecules (6.6 and 7.4 kJ mol(-1) in AV; 7.3 and 7.0 kJ mol(-1) in HMAP) suggest that the acetyl torsion is only slightly affected by intramolecular hydrogen bonding. The absence of torsional tunnellings due to the methoxy group indicates torsional barriers above 10.2 and 8.9 kJ mol(-1) for AV conformers, 10.1 and 10.4 kJ mol(-1) for HMAP. Conformational ratios and relative free energies have been estimated from relative intensity measurements of the spectral lines. Ab initio (MP2) and density functional calculations using the recent M05-2X empirical functional have been used to aid the experimental work in describing the structures, internal rotation barriers and isomerization potentials.

Practical methods for including torsional anharmonicity in thermochemical calculations on complex molecules: The internal-coordinate multi-structural approximation

Many methods for correcting harmonic partition functions for the presence of torsional motions employ some form of one-dimensional torsional treatment to replace the harmonic contribution of a specific normal mode. However, torsions are often strongly coupled to other degrees of freedom, especially other torsions and low-frequency bending motions, and this coupling can make assigning torsions to specific normal modes problematic. Here, we present a new class of methods, called multi-structural (MS) methods, that circumvents the need for such assignments by instead adjusting the harmonic results by torsional correction factors that are determined using internal coordinates. We present three versions of the MS method: (i) MS-AS based on including all structures (AS), i.e., all conformers generated by internal rotations; (ii) MS-ASCB based on all structures augmented with explicit conformational barrier (CB) information, i.e., including explicit calculations of all barrier heights for internal-rotation barriers between the conformers; and (iii) MS-RS based on including all conformers generated from a reference structure (RS) by independent torsions. In the MS-AS scheme, one has two options for obtaining the local periodicity parameters, one based on consideration of the nearly separable limit and one based on strongly coupled torsions. The latter involves assigning the local periodicities on the basis of Voronoi volumes. The methods are illustrated with calculations for ethanol, 1-butanol, and 1-pentyl radical as well as two one-dimensional torsional potentials. The MS-AS method is particularly interesting because it does not require any information about conformational barriers or about the paths that connect the various structures.

Impact of Molecular Conformation on Barriers to Internal Methyl Rotation: The Rotational Spectrum of m-Methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.

Density functional theory study of the structure and electric properties of poly(vinylidene cyanide trifluoroethylene) copolymer ultrathin film

The geometry, energy, internal rotation barrier, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene cyanide-trifluoroethylene) (P(VDCN-TrFE)) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level. The effects of chain length and the trifluoroethylene (TrFE) content on the copolymer chain stabilities, chain conformations, electric properties and vibrational spectra were examined and compared with those of the poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and the polyvinylidene cyanide (PVDCN) homopolymer to explore whether the ultrathin film of P(VDCN-TrFE) possess an expected good piezoelectricity or not. Based on the internal rotation potential curves of P(VDCN-TrFE) dimer models (H(CH 2C(CN) 2-CFHCF 2)H and H(C(CN) 2CH 2-CF 2CFH)H), the conformational angles, relative stabilities of α- and β-conformations and the transition energy barriers of β→α and α→β were discussed. The results show that the β-conformation is more stable than the α-conformation and the β→α transition in P(VDCN-TrFE) is more difficult than that in PVDCN. The energy difference per monomer unit between β- and α-chain P(VDCN-TrFE) copolymers with a variety of TrFE contents is smaller than that in PVDCN homopolymer, suggesting that α→β conformational change would be easier in the copolymer than in the homopolymer. The ideal β-chain of P(VDCN-TrFE) alternate copolymer with 50 mol% TrFE has a radius of about 16 Å, which is almost twice as large as PVDCN (8.5 Å) and the dipole moment contribution per monomer unit in the copolymer is even larger than the alternate P(VDF-TrFE) copolymer. The energy difference per monomer unit between the β- and α-chains decreases with increasing TrFE content. The contribution of average dipole moment per monomer unit in the β-chain is affected by the chain curvature and TrFE content, and there is a parabolic dependence on the VDCN content. The mean polarizability decreases with increasing TrFE content slightly. This work also predicted that there are some characteristic vibrational modes that may be used in identification of the α- and β-P(VDCN-TrFE) with different TrFE content.

Investigation on the potential barrier to internal rotation in molecules-Ab initio calculation and energy partition of the internal rotation barrier in complex molecule H3N-BH3

Based on the ab initio/6–31G* calculation, the potential barrier to internal rotation in molecule H3N-BH3 has been studied by means of PD/LSF atomic charge model and Buckingham(exp-6–1) energy partition method. The results indicate that the order of the contributions of the components to the total energy barrier ΔE is |ΔVed|(electrostatic)>|ΔV| (charge transfer)> |ΔVex| (exchange repulsion) > |ΔVdi| (dispersion). For ΔVes there are maxima at θ = 30° and 90°, and a saddle at θ = 60°. There are good linear relationships for the total barrier ΔE, ΔVex and ΔV| with cos3θ respectively, and the same for the dipole moment from PD/LSF model (μPD) and that from ab initio calculation (μQM) vs. cos3θ respectively.

ChemInform Abstract: Rotational Spectra of p-Isopropylbenzaldehyde: Assignment of Two Conformers and Observation of Torsionally Excited States.

Abstract Rotational spectra of p -isopropylbenzaldehyde were obtained at a very low temperature in a pulsed-beam Fourier transform-microwave spectrometer and over a range of much higher temperatures in a continuous wave spectrometer. Two species with ground-state rotational constants A =2975.15(14) MHz, B =550.1631(1) MHz, C =516.3000(1) MHz and A =3183.80(30) MHz, B =538.8124(1) MHz, C =512.0049(1) MHz were observed in the beam and are consistent with structures having the methine hydrogen coplanar with the benzene ring and respectively anti and syn to the carbonyl oxygen of the para formyl group. Relative intensities of the rotational transitions indicate that the two conformers have almost equal energies. Near room temperature, in addition to R-branch a -type band series consistent with the anti and syn conformers, a very intense band series with an intermediate B + C value is observed in the low-resolution microwave spectrum of p -isopropylbenzaldehyde, as previously reported [N.S. True, J. Phys. Chem., 87 (1983) 4622]. Temperature-dependent relative intensity measurements do not support the proposal of these authors that the intensity of this series reflects the population in torsional states above the torsional barrier and can be used to estimate the internal rotation barrier height. Additional factors not directly related to the free rotor population must contribute to the observed intensity.

Severe Energy Costs of Double Steric Interactions: Towards a Molecular Clamp

AbstractThe factors determining the ease of rotation about carbon–carbon single bonds connecting two internally rigid fragments such as phenyl, indenyl, anthracenyl and triptycyl are analysed. The internal rotation barriers in these molecules have been estimated on the basis of kinetic data or variable‐temperature NMR measurements, and the crystal structures have been analysed in terms of steric strain. Computer simulation of the internal rotation indicates that the estimated Closest Approach Distance, CAD, between sterically interacting atoms of the two interconnected fragments can be a helpful parameter for evaluating their rotational freedom, but must be used with caution. Thus, the barrier to rotation of a 3‐indenyl moiety linked to the 9‐position of anthracene is very high (ΔG≠ ≈ 25 kcal mol–1) compared to that in 3‐indenyltriptycene (ΔG≠ ≈ 12 kcal mol–1) despite the fact that the nominal CADs in both cases are very similar. Moreover, dimeric 2‐methylindenyl fragments linked by a single bond at the 3‐position undergo relatively slow rotation (ΔG≠ ≈ 14–15 kcal mol–1) owing to the simultaneous close approach of two pairs of sterically interacting hydrogens. Although the rotational barrier for a 2‐indenyl or phenyl moiety attached to the bridgehead atom of triptycene, or to the related dibenzobicyclo[2.2.2]octane system, is relatively low (ΔG≠ ≈ 8–9 kcal mol–1), further extension of the bridge to dibenzobicyclo[2.2.4]dioxadecane leads to an activation energy barrier in excess of 23 kcal mol–1, attributable to an intramolecular simultaneous “clamping” of the phenyl rings by the edges of the aromatic rings of the dibenzobicyclo[2.2.4]dioxadecane moiety. The X‐ray crystal structures of 15 molecules, including mono‐ and di‐indenyl‐anthracenes, racemic‐ and meso‐2‐methylindenyl dimers, phenyl‐ and indenyl‐triptycenes and ‐barrelenes, are reported.

The molecular structure of difluoroisocyanato silane: A combined microwave spectral and theoretical study

The microwave (MW) spectrum of HF 2SiNCO (1) has been obtained and analyzed in the I r representation for C S symmetry. The rotational constants (RC) are: A 7111.28104(179), B 1565.77581(49) and C 1347.52275(77) MHz; the centrifugal distortion constants are: Δ J 2.661(11), Δ JK 455.44(25), δ J 0.4237(51), ϕ K −54.96(29) kHz; the 14N nuclear quadrupole coupling constants are: χ aa +1.8833(27) and ( χ bb − χ cc ) −0.0214(58) MHz. The low value for ( χ bb − χ cc ) implies a nearly linear SiNC structure, while the experimental A value is only consistent with cis-HSiNC and trans-SiNCO orientations. The MW analysis was assisted by calculations of the equilibrium structure. Whilst small bases give a linear SiNCO skeleton, a 6-311G ++ (3df,3pd) basis set calculation shows that the molecule is quasi-linear for all four methodologies: DFT, MP2, MP4 or CCSD(T). However, all these methods find the lowest energy conformer has trans-HSiNC and trans-SiNCO dihedral angles, although the energy difference is very small. The internal rotation barrier for the HF 2Si group is less than 1 cm −1, with the lower energy conformer having a trans-HSiNC moiety. The calculated SiNC angles for the cis-HSiNC conformer are: 171.2 (B3LYP), 167.7 (MP2), 154.9 (MP4) and 154.6° (CCSD(T)), with differences up to 9° (MP2) in the trans-series. The potential energy (PE) surface for SiNC bending ( x) is unsymmetrical, but the differences from a symmetric form are very small. A B3LYP study leads to a polynomial fit of the SiNC angle ( x, radians) with the energy ( E, cm −1), where E = 1993(536) x 2 − 422(134) x 4 + 27(10) x 6 − 0.5(2) x 8; the alternating signs indicate a double minimum potential. We have re-determined the PE surfaces for silyl group rotation for several other isocyanates using similar methods.

Tin(II) Phthalocyaninate and Tin(IV) bis-Phthaloc Yaninate: A Quantum Chemical Study

The structural parameters of tin(II) phthalocyaninate PcSn and tin(IV) bis-phthalocyaninate Pc2Sn as well as of their cations are determined by B3LYP/SDD and PBE0/SDD quantum chemical methods. The PcSn molecule is characterized by C4v symmetry, and SnN bond lengths are 2.307/2.299 A (B3LYP/PBE0). The Sn nucleus is by 1.11 A (B3LYP, PBE0, single crystal X-ray diffraction analysis) higher than the plane of four neighboring nitrogen nuclei. The “hindered” configuration (D4d symmetry) with a high (27–30 kcal/mole) internal rotation barrier corresponds to the Pc2Sn energy minimum. The calculated equilibrium lengths of eight equivalent SnN bonds of 2.366/2.347 (B3LYP/PBE0) are similar to the average SnN bond length of 2.347 A (single crystal X-ray diffraction). Vertical and adiabatic ionization potentials are calculated: Iv 6.40/6.48 eV, IA 6.38/6.45 eV for PcSn and Iv 5.63/5.66 eV, IA 5.60/5.63 eV for Pc2Sn.

Cis– trans isomerization of carbon double bonds in monounsaturated triacylglycerols via generation of free radicals

We investigated the heat-induced cis/ trans isomerization of double bonds in monounsaturated lipids. When triolein (9- cis, 18:1) was heated around 180 °C, small amounts of isomerization products were obtained depending on the heating period. The heat-induced isomerization of triolein was considerably suppressed by the addition of different antioxidants or under nitrogen stream, and these additives simultaneously inhibited the thermal oxidation of double bonds in triolein. Therefore, an intermediate of the thermal oxidation reaction might be responsible for the heat-induced isomerization of the double bonds in triolein. The thermodynamics of the heat-induced isomerization of triolein (9- cis, 18:1) and trielaidin (9- trans, 18:1) were investigated using Arrhenius plot. The Arrhenius activation energies of cis double bonds in triolein and trans double bonds in trielaidin were 106 kJ/mol and 137 kJ/mol, respectively. The calculated internal rotational barrier heights of these double bonds were similar to those of the double bond of 2-butene radical and significantly lower than those of non-radicalized double bonds in 2-butene. These results suggest that heat-induced cis/ trans isomerization of triolein and trielaidin occurs mainly through the formation of radical species, which are the intermediates produced during thermal oxidation. The activation energy difference between the two forms suggests that trans trielaidin radicals are more stable than cis triolein radicals. The high thermodynamic stability of the trans double bonds in lipid radicals would influence the population of cis and trans isomers in edible oils and contribute to slight accumulation of trans-18:1 isomers during heating or industrial processing.

Molecular structure, conformational preferences and vibrational analysis of 2-hydroxystyrene: A computational and spectroscopic research

The molecular structure of 2-hydroxy-styrene has been investigated at DFT (B3LYP, mPW1PW91) and MP2 levels with an assortment of Pople’s and Dunning’s basis sets within the isolated molecule approximation. The presence of intramolecular hydrogen bonds has been theoretically characterized through a topological analysis of the electron density according to the Atom-In-Molecules, AIM, theory. The conformational equilibrium has been pursued by means of an analysis of the hydroxyl–phenyl and vinyl–phenyl internal rotation barriers. This analysis also allowed getting an insight into the effects governing the torsion barriers and the preferred conformations. A twofold scheme has been used for this goal, i.e. the total electronic energy changes and the natural bonding orbital, NBO, schemes. The vibrational spectrum was recorded and then calculated at DFT-B3LYP/6-31G∗ and cc-pVTZ levels. Two scaling methods, SQMFF and linear scaling, have been applied on the theoretical spectrum in order to analyse the experimental one. The results point out that at least three different conformers coexist at room temperature.

A quantum chemical study of the structure of fluorinated silver acetate(I) monomers and dimers

Quantum chemical calculations of the equilibrium structure, potential energy surface cross-sections along the nonrigid degrees of freedom of (CF3COOAg)2, (CHF2COOAg)2, (CH2FCOOAg)2, and (CH3COOAg)2 dimers and the corresponding monomers are presented. The B3LYP method with the augmented cc-pVTZ basis set for C, O, F atoms and the Stuttgart 1997 RSC basis set together with the relativistic effective potential for Ag atoms is used. It is shown that in all dimers the eight-membered ring is a rather rigid planar fragment, and the Ag atoms can be bonded with the bond order of 0.2 in dimers. Almost free rotation of acyclic groups around the C-C bond in (CH3COOAg)2 and (CF3COOAg)2 dimers transforms into a hindered one in the (CHF2COOAg)2 dimer, and further into the existence of syn- and anti-structures divided by a high rotation barrier in the (CH2FCOOAg)2 dimer. In monomers, the ratio of the internal rotation barriers is similar. With increasing number of fluorine substitution for hydrogen atoms in dimers the Ag-Ag bond length is found to increase (2.79 A, 2.81 A, 2.83 A, 2.84 A) and the ring rigidity to simultaneously decrease in the acetate-triflouroacetate series.

Molecular Structure of Trichloroethenylgermane, CH2═CH−GeCl3, as Studied by Gas-Phase Electron Diffraction. Experimental Determination of the Barrier of Internal Rotation of the Trichlorogermyl Group Supplemented with Quantum Chemical Calculations on CH2═CH−MX3 (M = C, Si, Ge, Sn, and X = H, Cl)

The molecular structure of trichloroethenylgermane, CH(2)=CH-GeCl(3), has been determined by electron diffraction and supported by quantum chemical calculations on CH(2)=CH-MX(3) (M = C, Si, Ge, Sn and X = H, Cl). An equilibrium syn conformation with C(s) symmetry is obtained both experimentally and theoretically where one of the Ge-Cl bonds eclipses the C=C bond. The barrier of internal rotation about the C-Ge bond is determined to be V(3) = 5.3(7) kJ mol(-1) using a dynamic model to simulate the internal motion. The most important structure parameters (estimated r(e)/A and angle/degree) are: r(C-Ge) = 1.911(5), r(C=C) = 1.345(5), r(Ge-Cl7) = 2.122(2), <C=C-Ge = 120.4(5), <C-Ge-Cl7 = 111.0(2), and <Cl7-Ge-Cl8) = 109.4(5) where the Cl7 atom is in the C=C-Ge plane and the Cl8 atom is out of the C=C-Ge plane. Uncertainties are estimated total standard deviations (sigma(tot)) given as: sigma(tot) = [sigma(2)(scale) + (2sigma(lsq))(2)](1/2) for bond lengths where sigma(scale) = 0.001r and sigma(lsq) is the least-squares standard deviation using a diagonal weight matrix and sigma(tot) = 2sigma(lsq) for the other parameters. The assignment of some of the fundamental frequencies has been discussed. The internal agreement between the calculated molecular geometry obtained from B3LYP and MP2(F) using the cc-pVQZ basis set and to the experimental geometry is not good, whereas the molecular geometry obtained from a CCSD/cc-pVTZ calculation is in very good agreement with experimental geometry. This investigation shows that the gas electron-diffraction method is capable of determining the rotational barrier for small suitable molecules with acceptable accuracy.

Exploring the G3 method in the study of rotational barrier of some simple molecules

AbstractThe G3 method was used to determine the internal rotation barriers of some simple molecules (H2O2, H2S2, N2H4, CH3OH, CH3NH2, and C2H6). The barriers were accurate with deviations lower than 0.5 kcal mol−1 with respect to the experimental data and comparable with other methods like CCSD(T)/aug‐cc‐pVTZ results. The G3 components showed that the MP4/6‐31G(d) energy is quantitatively improved according to: ΔEG3Large &gt; ΔE2df,p &gt; ΔE+ &gt; ΔEQCI. However, the relative energies showed that molecules containing lone pairs in neighbor atoms presented high barriers, which were particularly sensitive to polarization and diffuse functions. The G3Large and QCI effects showed no qualitative or quantitative relative contribution. Molecules containing lone pairs in one atom or only bond pairs presented low barriers and were sensitive not only to the polarization and diffuse effects but also depending on the improvement of the basis set through the G3Large correction. The QCI effect showed no qualitative or quantitative relevant contribution in any of the cases studied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The effect of deuterium substitution in the amino group on the internal-rotation barrier of acetamide

Peptide molecules XCO–NYY′ are characterized by low potential barrier V 3 to internal rotation of a methyl group substituted for X and/or Y. A most conspicuous example is acetamide, for which V 3 was previously reported to be 25.043857(19) cm −1 [8]. The present study intended to clarify why V 3 is so low in acetamide, by examining the effect of the out-of-plane bending or inversion of the amino group on the molecular structure through deuterium substitution for amino hydrogens. The potential barrier V 3 in acetamide was found to decrease by 2.630, 2.986, and 5.532 cm −1, when H′s at cis, trans, and both positions in the amino group were replaced by deuterium atoms, respectively. The reduction was proportional to the effective mass of the out-of-plane bending mode of the amino group (hereafter referred to as the amino inversion), which was in turn ascribed to the change in electronic resonance character of the peptide linkage. The amino inversion is coupled with the CH 3 internal rotation, producing an interaction term proportional to τ sin 3 α, where τ and α denote the amino inversion and methyl internal rotation angles, respectively. This coupling term, when the inversion is treated by second-order perturbation, yields a V 6 term in the internal-rotation potential function of the methyl group, in agreement with the finding of Ilyushin et al. [8], who derived an unusually large V 6 term of −10.044874(73) cm −1. It is quite interesting that even a small perturbation such as deuterium substitution causes a substantial change in electronic structure of the peptide linkage.

Large-amplitude vibrations of an N–H⋯π hydrogen bonded cis-amide–benzene complex

The ground-state N-H...pi interaction of 2-pyridone.benzene (2PY.Bz) has been studied by infrared-UV depletion spectroscopy of the supersonic-jet cooled complex [P. Ottiger et al., J. Phys. Chem. B (2009) 113, 2937]. Here, we investigate the large-amplitude vibrations of 2PY.Bz and its d(1)-2PY and benzene-d(6) isotopologues in the S(1) state, using two-color resonant two-photon ionization and UV-holeburning spectroscopies, complemented by RI-CC2 and SCS-RI-CC2 calculations of the S(1) state. The latter predict a tilted T-shaped structure with an N-H...pi hydrogen bond to the benzene ring, similar to the S(0) state. The binding energy is predicted to increase by 1.5 kJ mol(-1) upon S(1)<--S(0) excitation, in close agreement with the experimental value of 1.2 kJ mol(-1). The vibronic band structure up to 60 cm(-1) above the 0 band is dominated by large-amplitude delta tilting excitations, reflecting a change in the tilt angle of the T-shaped complex. The S(0) and S(1) state delta potentials were fitted to experiment, yielding a single minimum in the S(0) state and a double-minimum S(1) potential with delta(min) = +/-13 degrees. The second large-amplitude vibration is the theta twisting or benzene internal-rotation mode. Due to the C(6) symmetry of the benzene moiety the S(0) and S(1) state theta potentials are sixfold symmetric. Analysis of the theta band structure reveals that the S(0) and S(1)theta potentials are mutually aligned and that the internal rotation barriers are V(6)(S(0)) < 0.2 kJ mol(-1) and V(6)(S(1)) = 0.10(1) kJ mol(-1), in close agreement with the calculations. Weaker excitations of the totally symmetric intermolecular vibrations chi (shear), omega (bend) and sigma (stretch) vibrations are also observed. The 2PY intramolecular nu(1) overtone, corresponding to an 2PY amide out-of-plane twist distortion, lies approximately 30% higher than in bare 2PY, reflecting the hindrance of this motion by the strong N-H...pi interaction.

Conformational equilibria in vanillin and ethylvanillin

The conformational equilibria of vanillin and ethylvanillin have been investigated in a supersonic jet expansion using rotational spectroscopy. Two conformers have been detected for each molecule, with a dominant O-H···O intramolecular hydrogen bond locking the local conformation of the hydroxyl and methoxy/ethoxy groups. As a consequence, the observed conformers of vanillin differ only in the orientation of the aldehyde group, either cis or trans with respect to the methoxy group. For ethylvanillin the ethoxy group would plausibly generate additional trans (in-plane) or gauche (out-of-plane) orientations. However, the two detected conformations exhibit only planar ethoxy trans arrangements, with the gauche forms most probably depopulated by collisional relaxation in the jet. Torsional tunneling effects due to internal rotation of the terminal methyl groups were not detectable, indicating internal rotation barriers above 12.3 kJ mol(-1). The conformational population ratios in the jet have been estimated from relative intensity measurements. Ab initio (MP2) and DFT calculations using B3LYP and the recent M05-2X empirical functional supplemented the experimental work, describing the rotational parameters, conformational landscape and the aldehyde and methyl internal rotation barriers in these molecules.

A theoretical study of the effects of polar substitution on the activation barriers for internal rotation around the C-N bond in p-substituted nitrosobenzenes: comparison of DFT and MP2 calculations

The activation barriers for internal rotation around the C-N bond in p-substituted nitrosobenzenes were calculated using the density functional theory (DFT) and second-order Møller-Plesset (MP2) methods with the 6-31+g(d) basis set. The polarisable continuum model (PCM) was used to model the solvent effect. An explicit water molecule was also introduced to form a hydrogen bond with the nitrosogroup and its effect on the barrier was studied by DFT. The barriers were well-correlated with Hammett sigma^+ rather than sigma values, meaning a strong resonance effect. The MP2 method produces better and comparable results with the few available experimental values.

DENSITY FUNCTIONAL THEORY STUDY OF STRUCTURE AND ELECTRIC PROPERTIES OF POLY(VINYLIDENE CYANIDE TETRAFLUOROETHYLENE) COPOLYMER ULTRATHIN FILM

The geometry, energy, internal rotation barrier, vibrational spectra, dipole moments, and molecular polarizabilities of poly(vinylidene cyanide‐tetrafluoroethylene) (P(VDCN‐TeFE)) of α‐ and β‐chain models were studied with density functional theory at B3PW91/6‐31G(d) level. The effects of chain length and the tetrafluoroethylene (TeFE) content on the copolymer chain stabilities, conformations, electric properties, and vibrational spectra were examined and compared with those of the poly(vinylidene fluoride‐tetrafluoroethylene) (P(VDF‐TeFE)) copolymer and the polyvinylidene cyanide (PVDCN) homopolymer to explore whether the ultrathin film of P(VDCN‐TeFE) possesses an expected good piezoelectricity or not. The results show that the β‐conformation is more stable than the α‐conformation, the β→α transition is more difficult, and the α→β transition is easier in the copolymer than those in the homopolymer, predicting that a good piezoelectricity in the copolymer is hopeful. The contribution of average dipole moment per monomer unit in the β‐chain is affected by the chain curvature and VDCN content. The alternate P(VDCN‐TeFE) with 50 mol% TeFE has better piezoelectric properties than the alternate P(VDF‐TeFE). The mean polarizability decreases with increasing TeFE content. This work also predicted some characteristic vibrational modes that may be used in identification of the α‐ and β‐P(VDCN‐TeFE) with different TeFE content.