Equilibrium vs Ground-State Planarity of the CONH Linkage

Abstract

Planarity of the XC(d)NHY linkage has been investigated in unprecedented detail in a number of relatively simple compounds, including formamide (, and methyl carbamate (X ) OCH 3 , Y ) H). Reliable estimates of the equilibrium structures of formamide, cyanamide, acetamide, urea, carbamic acid, methylamine, dimethyl ether, and methyl carbamate are derived, mostly for the first time. It is shown that formamide, considered prototypical for the amide linkage, is not typical as it has a planar equilibrium amide linkage corresponding to a single-minimum inversion potential around N. In contrast, several molecules containing the CONH linkage seem to have a pyramidalized nitrogen at equilibrium and a double-minimum inversion potential with a very small inversion barrier allowing for an effectively planar ground-state structure. Observables of rotational spectroscopy, including ground-state inertial defects, quadrupole coupling and centrifugal distortion constants, and dipole moment components, as well as equilibrium CdO and C-N bond lengths are reviewed in their ability to indicate the planarity of the effective and possibly the equilibrium structures.

Quantum-chemical calculations of the structure, vibrational spectra, and torsional and inversion potentials of methylcarbamate

“…Table 4 shows that the calculation using B3LYP/cc-pVDZ with an anharmonic approximation was in general adequate to reproduce the experimental values of the vibrational frequencies and the intensities of methylcarbamate. However, the assignment of vibrations at medium and low frequencies did not always agree with the literature [13,18]. Thus, it should be noted that our quantum-chemical calculations predicted significant delocalization of the urethane vibrations.…”

“…The molecular fragment is delocalized. The average value of the corresponding dipole-moment component is equal to zero [13]. A calculation of a system of inversion states [14] confirmed such a point of view.…”

“…The hypothesis of a pyramidal structure was supported by high-level quantum-chemical calculations [13,14], the presence of a similar structure for a whole series of analogs [13] (cyanamide, acetamide, urea, etc. ), and certain experimental data.…”

“…Whether the amino group in methylcarbamate is planar or pyramidal has been discussed for a long time [13][14][15]. The hypothesis of a pyramidal structure was supported by high-level quantum-chemical calculations [13,14], the presence of a similar structure for a whole series of analogs [13] (cyanamide, acetamide, urea, etc.…”

Computational Study of the Electron Spectra of Acetamide and N-methylformamide

“…First, it is one of the best models of the peptide bond; secondly, there is the question of whether or not the arrangement of the bonds around the nitrogen atom is planar or pyramidal, the question being common to other nitrogen-containing molecules such as amines, amides, and nitro molecules; [4] thirdly, pyramidization aside, there is the question of the planarity of the heavy atom framework C(CO)N; [5] furthermore, the rotation of the methyl group leads to three conformers; [4] and finally the low methyl rotation barrier leads to an ambiguity of nonplanar equilibrium structures versus an effective planar ground state. [6] Demaison et al [6] made an extensive study of the planarity of the CONH linkage in formamide, cyanamide, acetamide, urea, carbamic acid, methylamine, dimethyl ether, and methyl carbamate. They concluded that the CONH linkage is quite complex.…”