CH Bond Lengths Research Articles

The ground-state average structure of methyl isocyanide

The use of recently determined highly precise inertial data for various isotopic modifications of methyl isocyanide has enabled the ground-state average, or rz, structure to be determined to within very narrow limits. Harmonic corrections to ground-state rotational constants have been calculated using a high-quality, experimentally determined harmonic force field. The derived zero-point inertial constants are sufficiently accurate to enable changes in the CH bond length and NCH bond angle on deuteration to be determined. The present rz structure determination is believed to be a physically realistic estimate of the ground-state average geometry of methyl isocyanide.

Born–Oppenheimer energy surfaces of similar molecules: Interrelations between bond lengths, bond angles, and frequencies of normal vibrations in alkanes

CH bond lengths, HCH and HCC bond angles, and CH symmetric and asymmetric stretching frequencies in alkane molecules are placed into four groups according to their occurrence in CH4, –CH3, CH2, and –CH, and are seen to vary in a regular fashion. The physical rationale offered for these variations relates them to balanced interactions between adjacent orbitals of CH and CC bonds, which are assumed to be common to all energy surfaces of alkane molecules. The regular variations are quantitatively reproduced by a consistent force field of alkanes, which in place of the usual harmonic stretching potentials uses only two Morse potentials, one for the CH bond, common to all four groups, and one for the CC bond. The correlated variation in bond lengths and bond angles, due to orbital interactions, is represented mainly by stretch–bend, stretch–stretch, and bend–bend cross terms. The resulting stretching frequencies, being dependent upon the second derivative of the Morse function, decrease with increasing bond length. The new force field yields bond lengths, bond angles, and vibrational frequencies, and reproduces the observed trend in their variation, mostly to within experimental accuracy. Remaining deviations are attributed to vicinal and higher order nonbonded interactions. Methane is included as a member of the alkane family and the new force field accounts successfully for its vibrational frequencies.

The overtone spectrum of acetylene: A rotational analysis based on a local model description

The local mode model is extended to account for the rotational structure in the CH-stretching overtone spectrum of acetylene from ΔvCH = 2 to 5. A local mode Hamiltonian, which includes nonrigidity and the resultant effects of rotation–vibration interaction, is derived and used as the basis for the analysis. The analysis indicates a strong interaction between CH bending and stretching which decreases as vCH increases. The change in equilibrium CH bond length with increasing vCH is obtained and an interesting anomaly at ΔvCH = 5 is identified.

ChemInform Abstract: CH Stretching Frequencies and Bond Strengths, and Methyl Group Geometry in CH3CXO Compounds (X: H, Me, F, Cl, Br, I, CN, OMe) and CH3CH2CN.

Abstract Infrared spectra in the CH stretching region are reported for CHD 2 CXO and CH 3 CXO compounds, where X = F, Cl, Br, I, CN, OMe, and for CHD 2 CD 2 CN and CD 3 CHDCN. In all the carbonyl compounds except the iodide the two out-of-plane CH bonds in the methyl group are significantly weaker than the in-plane one. Differences in CH bond length of up to 0.006 A are predicted, which are considered to be more reliable than the available microwave data. The separations of v as CH 3 and v CHD 2 , frequencies are compatible with a strong angular asymmetry (∠H s CH a ≠ ∠H a CH a ) in the acetyl compounds. The gauche and trans effects of halogen on v CH is are similar, but larger, than those in alkyl halides. In ethyl cyanide, the methyl CH bonds are identical in strength. The α and β substituent effects from the CN group fall into no simple pattern.

The ultraviolet absorption spectrum of thioformaldehyde

The spectra of H 2CS and D 2CS were surveyed over the wavelength region from 230 to 180 nm and four distinct absorptions were identified. These are assigned to transitions from the X ̃ 1A 1 ground state to the B ̃ 1A 1(π, π ∗) , C ̃ 1B 2(n, 3s) , D ̃ 1A 1(n, 3p y) , and E ̃ 1B 2(n, 3p z) electronic states. A vibrational and rotational analysis of the second system was undertaken. The results indicate that the molecule is planar in the C ̃ 1B 2(n, 3s) state and that while the CH and CS bond lengths remain near their ground-state values, the HCH angle increases substantially.

CH stretching frequencies and bond strengths, and methyl group geometry in CH 3CXO compoundS (X = H, Me, F, Cl, Br, I, CN. OMe) and CH 3CH 2CN

Infrared spectra in the CH stretching region are reported for CHD 2CXO and CH 3CXO compounds, where X = F, Cl, Br, I, CN, OMe, and for CHD 2CD 2CN and CD 3CHDCN. In all the carbonyl compounds except the iodide the two out-of-plane CH bonds in the methyl group are significantly weaker than the in-plane one. Differences in CH bond length of up to 0.006 Å are predicted, which are considered to be more reliable than the available microwave data. The separations of v as CH 3 and vCHD 2, frequencies are compatible with a strong angular asymmetry (∠H sCH a≠ ∠H aCH a) in the acetyl compounds. The gauche and trans effects of halogen on v CH isare similar, but larger, than those in alkyl halides. In ethyl cyanide, the methyl CH bonds are identical in strength. The α and β substituent effects from the CN group fall into no simple pattern.

Isolated CH stretching frequencies and bond properties in some organo-germanes and methyl arsines

Infrared spectra in the CH stretching region have been obtained from partially deuterated methyl, dimethyl and trimethyl germanes and arsines, ethyl germane, MeGeH 2X, where X = F, Cl, Br, I and MeGeHCl 2. In general, the isolated CH stretching frequencies show patterns of behaviour with respect to the α and β substituent effects of CH 3, GeH 3 and halogen groups similar to these found in the corresponding carbon compounds, although on a smaller scale. However, the trans effect of halogen is almost independent of the nature of the halogen, and a CH bond trans to GeH 3 is weaker than one trans to CH 3. In methyl arsine, an anomalous lone pair trans effect occurs, as previously found in methyl phosphine. CH bond lengths and dissociation energies, and average HCH angles are predicted for these compounds.

The molecular structures of styrene and 4-bromostyrene and the deuterium quadrupole coupling constants of styrene-d8determined from the proton and deuterium magnetic resonance spectra of nematic solutions

The proton spectra of styrene and 4-bromostyrene as solutes in liquid crystalline solvents have been analysed to yield sets of dipolar coupling constants. It is found that for both molecules the dipolar couplings are consistent with planar ground state structures only if the torsional motion of lowest frequency occurs at about 80 cm-1, in contrast to values of around 40 cm-1 assigned to this mode by other methods. The structure of the olefin fragment, assuming rr CH bond lengths of 1.08±0.01 A, has r(CC) = 1.31 ± 0.06 A (styrene) and 1.32 ± 0.06 A (bromostyrene) and CCC bond angles of 136 ± 6° (styrene) and 134 ± 6° (bromostyrene). The deuterium spectrum of styrene-d 8 has been analysed under the same conditions of concentration and temperature as the proton spectrum and the results used to derive the quadrupole coupling constant for the aromatic CD bonds as 186 ± 2 kHz with an asymmetry parameter of 0.03 ± 0.03. These same properties for the olefinic CD bonds are derived to be 181 ± 9 kHz and -0.01±0.03 ...

Calculated barriers to abstraction and exchange for CH4+H

Saddle point geometries and barrier heights have been calculated for abstraction and exchange in CH4+H using POL–CI wave functions with basis sets up to triple zeta valence with double zeta polarization on C and single zeta polarization on H. The saddle point for the abstraction reaction is found to have C3v symmetry (abstraction collinear with a CH bond). The calculated saddle point geometry is closer to products (CH3+H2) than to reactants (CH4+H) and has the CH and HH bonds stretched by 0.38 Å (35%) and 0.18 Å (24%), respectively. The CH bond lengths of the remaining three CH bonds change only very slightly (<0.01 Å) during the reaction, while at the saddle point each methyl CH bond is ∼12.4° out of the plane perpendicular to the C–––H–––H axis and containing the C atom (19.47° for CH4 and 0° for CH3). The calculated barrier height is 15.9 kcal/mole (using the largest basis set). Comparison to comparable calculations for H3 indicates an error of ∼2.4 kcal/mole due to the POL–CI approximation leading to 13.5 kcal/mole as the best estimate of the barrier height. The saddle point for the exchange reaction has a trigonal bipyramidal geometry. The calculated saddle point geometry has rCH=1.41 Å (equal CH bond lengths), while the methyl CH bond lengths change only slightly (<0.01 Å) during the reaction. The calculated barrier height is 36.9 kcal/mole (for the largest basis set). Using the same estimate for the error due to the POL–CI approximation as for the abstraction reaction leads to 34.5 kcal/mole as the best estimate of the barrier height for the exchange reaction.

Calculated barrier to hydrogen atom abstraction from CH4 by O(3P)

The barrier height and transition state geometry have been calculated for the reaction CH4+O(3P) →CH3+OH using POL–CI wave functions with a valence double zeta plus polarization basis set. The saddle point geometry is found to be of C3v symmetry with the CH and OH bonds stretched by ∼0.27 Å (25%) and ∼0.21 Å (21%), respectively. The CH bond lengths of the CH3 group change only very slightly (<0.01 Å) during the reaction, while at the transition state each CH bond is ∼13.1° out of the plane perpendicular to the CHO axis and containing the carbon atom (19.5° for CH4 and 0.0° for CH3). The calculated barrier height is 14.4 kcal/mole using a [3s2p1d/2s1p] basis set. From comparison to similiar calculations for O(3P)+H2→OH+H the basis set error in ΔEb is estimated to be ∼2.4 kcal/mole leading to a predicted barrier height of ∼12.0 kcal/mole. Including zero point corrections leads to estimated activation energies of 10.3 kcal/mole (3A′) and 10.1 kcal/mole (3A″) as compared to experimentally derived activation energies of 9.0–11.4 kcal/mole.

Microwave spectrum, structure, and internal rotation of chloromethylmethylether

Microwave spectra of chloromethylmethylether and its 21 isotopically substituted species were measured. The r s structure of this molecule was determined from the observed moments of inertia under several assumptions which were necessary to avoid the use of unreliable r s coordinate values. Structural parameters obtained for this molecule are roughly close to those reported by Ikeda et al. [ J. Mol. Spectrosc. 53, 101 (1974)]. However, one of three CH bond lengths for the methyl group exhibits a remarkably high value (1.130 Å). From the structure obtained, the barrier to internal rotation of the methyl group was calculated taking into account the coupling effect with the skeletal torsion.

HCNO as a semirigid bender

The semirigid bender Hamiltonian [Bunker and Landsberg, J. Mol. Spectrosc., 67, 374–385 (1977)] is used to fit the rotation-vibration energy level separations in the fulminic acid (HCNO) molecule. The allowance made in the model for the variation of the CH and CN bond lengths with the HCN bending angle proves to be very important, and as well as achieving a good fit we are able to make a detailed investigation of the shape of the HCN bending potential function. From the results we conclude that the equilibrium structure of HCNO is linear but that excitation of the ν 1 or ν 2 stretching vibrations gives rise to an effective HCN bending potential function having its minimum at a nonlinear configuration. Even in the ground state the zeropoint vibrational contributions from ν 1 and ν 2 to the effective HCN bending potential give a small barrier (11.5 cm −1) to linearity, and we determine that the zero-point HCN bending vibrational amplitude is ±34°.

Isolated CH stretching frequencies and bond properties in CH 3CF 3, CH 2(CF 3) 2 and CH(CF 3) 3

The CH stretching frequencies recently reported in CHD 2CF 3, CHD(CF 3) 2 and CH(CF 3) 3 are used to show that the influence of an αCF 3 group on CH bond strength is irregular and very different from that of αCH 3. The value in CHD 2CF 3 is slightly higher than expected from the β-effects of individual CF bonds as found in EtF. An attempt is made to discuss the strength variations in terms of the Bellamy—Mayo model. Predictions are made of CH bond lengths and dissociation energies throughout, and ∠HCH in CH 3CF 3 is estimated.

Isolated CH stretching frequencies, bond strengths and lengths in some mep and MeS compounds

Infrared spectra in the CH stretching region have been obtained for CHD 2PH 2, (CHD 2) 2PH, (CHD 2) 3P and (CHD 2) 2S which enable small differences in CH bond strength and length to be discerned within each methyl group of each compound. Small “normal” trans effects of the lone pair of electrons are found in Me 3P and Me 2S, compared with essentially zero ones in Me 2PH and MeSH and a small “abnormal” effect (CH trans to lone pair stronger than CH trans to PH) in MePH 2. Ab initio calculations of r eCH using the 4-31G basis set have been made for the above molecules and for Me 3N. For the latter, the difference in r eCH within the methyl group is calculated to be 0.015 Å, in excellent agreement with the predicted Δ r OCH value of 0.016 Å. The smaller differences Δ r OCH in the MeP and MeS compounds are not well reproduced, r eCH 1p is consistently smaller than or equal to r eCH H or Me , in agreement with the negative trans effect found in MePH 2, but the influence of Me substitution in reversing this is not predicted.

Cubic force constants and equilibrium geometry of methane from Hartree–Fock and correlated wavefunctions

The ab initio harmonic and cubic force field of methane has been determined, both from Hartree–Fock and from highly correlated CEPA wavefunctions. The effect of correlation is small on the force field, particularly on the cubic constants. The ab initio cubic force constants were used to calculate the equilibrium CH bond length from the electron diffraction rg and from the spectroscopic r0 values. The weighted mean of the results is re=1.0862 ±0.0005 Å. There remains a small but disturbing discrepancy between the best ab initio and the present corrected experimental results, both for the bond length and for the CH stretching force constants.

Theoretical study on geometry and spectroscopic properties of 1,1′-binaphthyl in the electronic ground and first excited singlet states

Equilibrium geometries for the electronic ground and first excited singlet states of 1,1'-binaphthyl have been calculated by minimization of the total energy with respect to all internal coordinates. Using these results, an interpretation of the fluorescence S 1→ S 0 and absorption spectra S m ← S 0 and S n ← S 1 in rigid and fluid solutions is given. For the first time the equilibrium geometry of the first excited singlet state of 1, 1′-binaphthyl has been calculated. On excitation to the S 1 state the dihedral angle θ between the two naphthalene moieties is de- creased from 61 ° to 41 °. A detailed survey of CH bond lengths in the S 0 and S 1 states has been given. This result should be of particular importance for the theoretical treatment of radiationless transitions. Using equilibrium geometries for the S 0 and S 1 states a satisfactory interpretation of the S m ← S 0 and S n ← S 1 absorption spectra as well as of the fluorescence spectra in fluid and rigid solutions can be given. Concerning the S n ← S 1 absorption spectrum in fluid solution, the calculations predict a strong absorption (A ← B transition) in the still uninvestigated region of energies lower than 11000 cm −1. From the results of this paper and of other calculations it can be concluded that the Warshel-Karplus method yields reliable equilibrium geometries for electronic ground and excited states of unsaturated hydrocarbons [22,23].

Ab initiostretching force constants for HCCH, FCCH and C1CCH

Harmonic and anharmonic stretching force constants have been calculated for HCCH, FCCH and C1CCH from ab initio SCF wavefunctions by Pulay's force method. The ab initio harmonic and diagonal anharmonic force constants show good agreement with a recent experimental quartic force field for acetylene, while most of the force constants for FCCH and C1CCH stand as predictions. All the off-diagonal force constants (in internal valence coordinates) are calculated to be very small and thus of practically no influence on observable quantities. From systematic errors in the calculations, estimates are given for the CH and CC equilibrium bond lengths of FCCH and C1CCH.

The infrared spectrum of CHD 2NC and the ground state structure of methyl isocyanide

Analysis of the rovibration spectrum of CHD 2NC has enabled the rotation parameter ( A 0 B 0) to be determined. Knowledge of the microwave B 0 and C 0 constants enables A 0(CHD 2NC) to be determined, which, when used in conjunction with all other B 0 and C 0 constants for isotopic methyl isocyanides, allows the ground state structure to be defined within fairly narrow limits. The CH bond length is much shorter than that determined by the substitution technique (r s structure). It is now in almost perfect agreement with the value predicted by the CH vibration frequency in CHD 2NC, and falls in the range of CH bond lengths exhibited by all other symmetric top methyl group molecules.

CH stretching frequencies, bond strengths and the effects of internal rotation in some CH 3-boron compounds

Infrared and Raman spectra in the CH and CD stretching regions are reported for the CH 3, CHD 2 and CD 3 species of MeBF 2, MeBCl 2, Me 3B and (MeBO) 3. The appearance of pairs of CH stretching bands in the i.r. spectra of all the CHD 2 species, together with splittings in ν aCH 3 and ν aCD 3 frequencies, indicate a CH bond strength variation with internal rotation angle. The i.r. bands in the gas phase of CHD 2BF 2, CHD 2BCl 2 and (CHD 2BO) 3 represent ν∥CH and ν⊥CH respectively, as expected for free internal rotation, in each case ν∥CH > ν⊥CH. By contrast in the Raman effect, the main CHD 2 frequency is a polarised line representing ν av.is CH = 1 2 (ν∥CH + ν⊥CH). The spectra in the gas phase of (CHD 2) 3B are slightly ambiguous, and two possible values for ν∥CH and ν av.is CH are quoted. In the crystal phases MeBF 2, and possibly also MeBCl 2, appear to exist in two conformations, with a CH bond either vertical (I) or parallel (II) relative to the skeletal plane, whereas (MeBO) 3 and Me 3B probably exist only in I. Analysis of all the frequencies is based on a model in which both principal and interaction force constants vary with cos 2φ, where φ is the internal rotation angle. This shows the splittings of ν aCH 3 and ν aCD 3 in the gas phase to be anomalous. Predictions of CH bond lengths, dissociation energies and HCH angles are made, which are in excellent agreement with recent experimental and ab initio results.

Excited states and photochemistry of saturated molecules. ab initio calculations on methane

Abstract STO 4G calculations are carried out on methane in both tetrahedral and Jahn—Teller distorted C 2v and C 3v symmetries. Large increases in CH bond lengths are predicted in all states, and the I B 1 state in C 2v symmetry is predicted to dissociate to I B 1 methylene and 1 A 1 H 2 width no energy barrier. THe results are compared with earlier INDO calculations, and the latter are found to qualitatively consistent with the ab initio results.