Microwave Spectra, Planarity, and Conformational Preferences of cis- and trans-N-Vinylformamide

Abstract

The microwave spectra of a mixture of cis- and trans-H-N-C-O forms of N-vinylformamide, (H(2)C═CHNHC(═O)H), have been measured at room temperature in the 18-75 GHz spectral range. The spectra of two forms were assigned. The first of these forms has a cis arrangement for the H-N-C-O chain of atoms, whereas the second form has a trans arrangement. The C-C-N-C chain of atoms is antiperiplanar (180°) in both forms. The inertial defect of the ground vibrational state of cis is -0.142(5) × 10(-20) u m(2), whereas this parameter is -0.087098(26) × 10(-20) u m(2) for trans. It is concluded that the equilibrium structures of both cis and trans are completely planar. The dipole moment determined from Stark effect measurements is μ(a) = 9.96(8), μ(b) = 2.22(3), μ(c) = 0 (by symmetry), and μ(tot) = 10.20(8) × 10(-30) C m [3.06(2) D], for cis, and μ(a) = 7.64(16), μ(b) = 9.24(10), μ(c) = 0 (by symmetry), and μ(tot) = 12.0(2) × 10(-30) C m [3.59(5) D] for trans. The spectrum of one vibrationally excited state, presumably the first excited state of the torsion about the C-N bond of cis, was assigned and the frequency of this state was determined to be 76(15) cm(-1) by relative intensity measurements. The spectra of two vibrationally excited states of trans were assigned. These states are assumed to be the first excited state of the torsion about the C-N bond, and a low bending vibration. Relative intensity measurements yielded 101(20) and ca. 300 cm(-1), respectively, for the frequencies of these normal vibrations. Accurate values of the quartic centrifugal distortion constants, the dipole moments, and the vibration-rotation constants have been obtained for both cis and trans. The experimental work has been augmented by high-level quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD(T)/cc-pVTZ levels of theory. The theoretical calculation performed without symmetry restrictions correctly predict that cis and trans are both planar. The CCSD(T) rotational constants are in excellent agreement with their experimental counterparts, whereas the B3LYP quartic centrifugal distortion constants and the vibration-rotation constants are in fairly good agreement with experiments. The CCSD(T) dipole moments deviate more than expected from the experimental dipole moments. It is estimated that further conformers of cis and trans must be at least 4 kJ/mol higher in energy.

14N quadrupole coupling in the microwave spectra of N-vinylformamide

“…We believe that this change depends on the increasing number of alkyl substitutions attached to the nitrogen atom. In the second group, the two conformers of N-vinylformamide (6) possess almost exactly the same χ zz value, which is similar to that of Nphenylformamide (7) but different from those of the saturated formamides (2 -5), and gradually decreasing in magnitude from 3.8510(11) MHz in formamide (1) to 3.41 (7) MHz in diformamide (8). It is likely, in this second group, that π-electron conjugation from the substituent through the lone pair electrons of the nitrogen to the carbonyl bond affects the EFGs at the nitrogen nucleus, which affect is manifest in significant variation of the χ zz constant.…”

“…At the beginning of the experimental work, some microwave transitions of cis and trans conformers given in Ref. [8] were remeasured with higher resolution using a molecular beam Fourier transform microwave (MB-FTMW) spectrometer operating in the frequency range from 26.5 GHz to 40 GHz [13]. N-Vinylformamide was purchase from TCI Deutschland GmbH, Eschborn, Germany, with a stated purity of over 98 % and used without further purification.…”

“…The simplest amide, formamide [1], as well as N-methylformamide [2], N-ethylformamide [3], N,N-dimethylformamide [4], and N,N-methylethylformamide [5] were thoroughly studied. Reports on the microwave spectra of three unsaturated formamides, N-phenylformamide [6,7], N-vinylformamide [8], and diformamide [9] are available in the literature. Such unsaturated molecules are very interesting, since π-electron conjugation between the double bond(s), the lone electron pair of the nitrogen, and the carbonyl bond is possible.…”

“…The microwave spectrum of N-vinylformamide, HC(=O)N(H)CH=CH 2 , was recently investigated by Møllendal and Samdal [8], at room temperature from 18 to 75 GHz. Spectra attributed to the trans and cis forms shown in Fig.…”

Conformational Properties of cis- and trans-N-Cyclopropylformamide Studied by Microwave Spectroscopy and Quantum Chemical Calculations

“…We have for these reasons for a long time taken an interest in gas-phase studies of them. Our previous MW and GED studies include cis - and trans - N -vinylformamide (H 2 CCHNHC­(O)­H), acetamide (CH 3 CONH 2 ), 2-fluoroacetamide (CH 2 FCONH 2 ), , 2-chloroacetamide (CH 2 ClCONH 2 ), , 2-iodoacetamide (CH 2 ICONH 2 ), 2,2-difluoroacetamide (CF 2 HCONH 2 ) 2,2-dichloroacetamide (CHCl 2 CONH 2 ), 2-chloro-2,2-difluoroacetamide (CF 2 ClCONH 2 ), 2,2,2-trifluoroacetamide (CF 3 CONH 2 ), 2,2,2-trichloroacetamide (CCl 3 CONH 2 ), propionamide (CH 3 CH 2 CONH 2 ), formic hydrazide (H 2 NNHCHO), acrylamide (H 2 CCHCONH 2 ), methyl carbamate (CH 3 OCONH 2 ), , methoxyacetamide (CH 3 OCH 2 CONH 2 ), and 2-azetidinone. , Experimental contributions from other laboratories include, for example, the prototype formamide (HCONH 2 ), − urea (H 2 NCONH 2 ), cis - and trans - N -methyl formamide (HCONHCH 3 ), N,N -dimethylformamide (HCON­(CH 3 ) 2 ), C–N–C–O- cis -N-ethylformamide (HCONHCH 2 CH 3 ), O–N–C–O- trans -methoxyformamide (HCONHOCH 3 ), C–N–C–O- cis -formanilide (C 6 H 5 NHCHO), , acetamide, − C–N–C–O- cis - N -methylacetamide (CH 3 CONHCH 3 ), C–N–C–O- cis - N -methylpropionamide (CH 3 CONHCH 2 CH 3 ) C–N–C–O- trans -acetanilide (CH 3 CONHC 6 H 5 ), and alaninamide (CH 3 CH­(NH 2 )­CONH 2 ) . Rotation isomerism is possible in several of the compounds listed above, but acrylamide is the only example of an amide where the MW spectra of more than one rotamer have been assigned …”