Molecular mechanics (MM4) study of amines

Abstract

The MM4 force field has been extended to include aliphatic amines. About 20 amines have been examined to obtain a set of useful molecular mechanics parameters for this class. The vibrational spectra of seven amines (172 frequencies) calculated by MM4 have an overall rms error of 27 cm(-1), compared with corresponding MM4 value of 24 cm(-1) for alkanes. The rms and signed average errors of the moments of inertia of nine simple amines compared with the experimental data were 0.18% and -0.004%, respectively. The heats of formation of 30 amines were also studied. The MM4 weighted standard deviation is 0.41 kcal/mol, compared with experiment. Electronegativity effects occur in the hydrocarbon portion of an amine from the nitrogen, and are accounted for by including electronegativity induced changes in bond lengths and angles, and induced dipole-dipole interactions in the molecule. Negative hyperconjugation results from the presence of the lone pair of electrons on nitrogen, and leads to the Bohlmann bands in the infrared, and also to strong and unusual geometric changes in the molecules (Bohlmann effect), all of which are fairly well accounted for. The conformational energies in amines appear to be less straightforward than those for most other classes of molecules, apparently because of the Bohlmann effect, and these are probably not yet completely understood. In general, the agreement between the MM4 calculated results and the available data is reasonably good.

Chirality effects in gas-phase spectroscopy and photophysics of molecular and ionic complexes: contribution of low and room temperature studies

“…Its coming into resonance with a vibrational transition of the probed species manifests itself by a depletion of this species' ground state hence a dip in the probe-induced signal. The difficulties due to accidental overlaps of S 0 -S 1 spectra of different species can be circumvented by the use of 160 A. Zehnacker multiple resonance techniques involving two or three IR lasers [123]. However, the IR-UV double-resonance techniques are limited to systems endowed with an aromatic chromophore unless VUV is used as a probe [124,125].…”

“…Force fields combined with powerful algorithms for the exploration of the PES have brought decisive contribution [154][155][156][157][158][159]. Today, polarisable force fields including n-body effects offer promising opportunities [160,161], especially when combining realistic energies with fast extensive exploration of the PES by replica exchange dynamics [159]. The calculation method used for local optimisation must meet the requirement that both hydrogen bonds and dispersion are accurately described, while keeping the calculation cost acceptable despite the size of the systems.…”

Molecular Mechanics Force Fields and their Applications in Drug Design

“…The MM4 work has been reported for saturated hydrocarbons, fluorinated hydrocarbon [39], alkenes, conjugated hydrocarbons, carbonyl compounds, amines [40] and sulfides. The improper torsion constants in MM4 are in units of kcal/mol and not in units of mdyne Å/rad 2 as in MM3.…”

Heats of Formation

“…Thus, the MM4 method has been extended to include alcohols/ethers, 47 amines, 48 sulfi des, 49 aldehydes/ketones, 50 carboxylic acids/esters, 51 and amides. One simply needs to determine a few bond energies and chain branching parameters for each different functional group, and also parameters that connect the functional group to the alkane fragment.…”