Methylamine, rotational barriers

Changing the atom bound to a methyl group from carbon to nitrogen to oxygen, as in going from ethane to methylamine to methanol, produces a decrease in the rotational barrier from 2.88 to 1.98 to 1.07kcal/mol. This closely approximates the 3 2 1 ratio of the number of H—H eclipsing interactions in these three molecules. 

Several calculations on amino and nitro compounds were performed as part of the validation of Tripos 5.252. Thus, the rotational barriers in methylamine and dimethylamine were calculated to be 2.8 and 4.9 kcalmol-1, respectively, in fair agreement with experiment (2.054 and 3.654 kcalmol-1, respectively), and the A-value of the nitro group was calculated to be 0.6 kcalmol-1, only about half of the experimental value... 

Rotational barriers. All the force fields examined here reproduce the experimentally observed increase in the rotational barrier on going from methylamine to dimethylamine and, in particular, MM2 and DREIDING do a good job in matching the experimental data. 

Semi-empirical models are markedly inferior to all other models dealt with (except the SYBYL molecular mechanics model) for barrier calculations. Major trends in rotation barriers are often not reproduced, for example, the nearly uniform decrement in rotation barrier from ethane to methylamine to methanol. None of the semi-empirical models is better than the others in this regard. One the other hand, AMI is clearly superior to MNDO and PM3 in accounting for nitrogen inversion barriers. All in all, semi-empirical models are not recommended for barrier calculations. 

The two-dimensional PES shown in Figure 8.17 (as well as in Figures 8.3b and 8.7c) is typical of internal rotation coupled to inversion of the other part of the system. This situation is also realized in methylamine inversion, where the rotation barrier is modulated not by a harmonic oscillation but by motion in a double-well potential. The PES for these coupled motions can be modeled as follows ... 

One of the difficulties encountered when investigating or discussing rotation barriers about C-0 and especially C-N bonds is that this process may be overshadowed by a competitive process, namely inversion. Inversion barriers for simple amines are in the region of 5 kcal/mol [86], thus somewhat above the rotation barrier reported in Table 7 for methylamine. However, bulky substituents may markedly increase the rotation barrier and cause inversion to become the preferred process of conformational interconversion. This is precisely what happens with ferf-butylamines, where inversion (LXXVa-LXXVb interconversion) but not C-N bond rotation (LXXVa-LXXVc interconversion) is observed [87],... 

The infrared and especially microwave spectra of methylamine and its deuterated species have been studied in considerable detail [see paper for further references]. The potential barriers to internal rotation and inversion are both relatively high [Table 6 internal rotation barrier is 684 cm in the ground state of CH3NH2] but the splittings of the energy levels are measurable. 

Inversion doubling has been observed in microwave spectrum of methylamine CH3NH2. This splitting depends on the quantum numbers of rotation and torsion vibrations [Shimoda et al., 1954 Lide, 1957 Tsuboi et al., 1964]. Inversion of NH2 alone leads to the eclipsed configuration corresponding to the maximum barrier for torsion. Thus, the transition between equilibrium configurations involves simultaneous NH2 inversion and internal rotation of CH3 that is, inversion appears to be strongly coupled with internal rotation. The inversion splits each rotation-vibration (n, k) level into a doublet, whose components, in turn, are split into three levels with m = 0, 1 by internal rotation of the... 

The magnitudes of the barriers to rotation of many small organic molecules have been measured. The experimental techniques used to study rotational processes include microwave spectroscopy, electron diffraction, ultrasonic absorption, and infrared spectroscopy. Some representative barriers are listed in Table 2.1. As with ethane, the barriers in methylamine and methanol appear to be dominated by hyperconjugative stabilization of the anti conformation. The barrier decreases (2.9 2.0 1.1) in proportion to the number of anti H-H arrangements (3 2 1). (See Topic 1.1 for further discussion.) ... 

Most barriers to internal rotation turn out to be repulsive dominant. Such is the case for methanol, methylamine, propane, propene, hydrazine, and, as has been seen, ethane and ethyl fluoride. Attractive dominant barriers are indicated for acetaldehyde, hydroxylamine, and hydrogen peroxide. 

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