C-H Bond length

Write and run an MM3 input file for methane from scratch, that is, open an empty file and put in all the necessary infomiation to do the MM3 calculation on CH4. What is the enthalpy of formation of CH4 What are the C—H bond lengths and angles ... 

Similarly, using another model based on heteroatom increments proposed by Dischler (267), it was possible to calculate a very satisfactory empirical geometry for thiazole (122) with the aid of Shoolery s correlation (268) between and C-H bond length. 

In this exercise, you will explore the bond rupture process by performing a potential energy surface scan. Run potential energy surface scans for these molecules, gradually increasing one of the C-H bond lengths, using the specified model chemistries ... 

As can be seen from Table I, the C-C bond distance as described by LDF is closer to experiment than the corresponding HF value obtained with a 6-3IG basis. Including correlation via second and third order Moller-Plesset perturbation theory and via Cl leads to very close agreement with experiment. The C-H bond length is significantly overestimated in the LDF calculations by almost 2%. The HCH bond angle is reasonably well described and lies close to all the HF and post-HF calculations. Still, all the theoretical values are too small by more than one degree compared with experiment the deviation from experiment is particularly pronounced for the semi-empirical MNDO calculation. 

As expected, the C-C bond lengA widens significantly for the rotational transition state. Here, the agreement between the semi-empirical MNDO results and the first-principles LDF results is remarkable. The discrepancy in the C-H bond length remains, but the trend of a small bond shortening from the ground state to the rotational transition state can be found for both the MNDO and the LDF calculation. 

Table 8-1. Basis set dependence of SVWN-optimized C-C/C-H bond lengths [A].

Schafer, L., and K. Siam. 1988b. Comment on Accuracy of Ab Initio C-H Bond Length Differences and Their Correlation with Isolated C-H Stretching Frequencies. J. Chem. Phys. 88, 7255-7256. 

Fig. 7.9 In the a-helical conformation of the model dipeptide N-acetyl N -methyl glycineamide (left), the C-H bond lengths and H-C-X angles at the a-carbon are different (1.081 A and 1.078 A for C-H and 109.3° and 109.7° for H-C-N). Thus, the a-carbon is asymmetric. In contrast, in the C5 conformation of N-acetyl N -methyl glycineamide (right), bonds and angles at C(a) are identical, there is a molecular symmetry plane, and the a-carbon is symmetric. (All values from Schafer et al. 1984.)...

CHD Re (CO) j. are very close to the gas phase positions and the shift (Mn to Re) is almost identical in gas phase and matrix. Thus, V(C-H). in the matrix will provide information on the C-H bond length. On generation of the unstable 1 2 complexes, there is a downward shift in V(C-H). of 11-14 cm implying a bond lengthening of. 0011-.00 X. This is clearly a very sensitive probe of the effect of slight electronic perturbation of the metal centre and is likely to be useful for other systems. 

Fig. 8. Average observed (top) and calculated (bottom) lengths (A), angles, and torsion angles (deg) in tri-t-butylmethane (14) (standard deviations in parentheses the average observed C-H bond length is 1.111 A, the calculated value 1.112)...

It has sometimes proved possible to determine the location of the hydrogen atoms in a given molecule by calculation when it could not for various intrinsic reasons be found by experiment. This was the case for dimethylsulfoxide (CH3)2SO (40). Although many experimental studies of its geometry in the gas phase had shown that the three (C—H) bond lengths within each methyl group were significantly different, it had proved impossible to determine precisely why. 

Table 31 The calculated C—H bond lengths for some neutral molecules and their conjugate bases at the 4-31G level."...

Molecule C—H bond length (A) Conjugate base C—H bond length (A)... 

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