Microwave Spectrum and Molecular Structure of Formamide

Abstract

The microwave spectra of ten isotopic species of formamide (H2N–CHO) have been investigated. Type a and type b transitions have been identified and measured for all ten species. The inertial defect is found to decrease whenever a heavier isotope is substituted for any atom in the NH2 group, and in fact is negative for four of the species studied. It is concluded that the molecule is nonplanar with the H2N–C group forming a shallow pyramid. The structural parameters deduced from the rotational constants are: r(N–H′, where H′ is trans to the aldehyde hydrogen) = 1.014±0.005 A, r(N–H″, where H″ is cis to the aldehyde hydrogen) = 1.002±0.005 A, r(N–C) = 1.376±0.010 A, r(C–H) = 1.102±0.010 A, r(C=O) = 1.193±0.020 A, ∠H′NH″ = 118°53′ ±40′, ∠H″NC = 120°37′±40′, ∠H′NC = 117°9′±40′, ∠NCO = 123°48′±40′, ∠NCH = 113°14′±40′ and ∠OCH = 122°58′±40′. The dihedral angles between the H′NC plane and the NCO plane, and between the H″NC plane and the NCH plane, are 7°±5°, and 12°±5°, respectively. In all the spectra investigated each line was accompanied by a vibrational satellite line of anomalously high intensity. Measurements of relative intensities were carried out for H2N14–CHO, cis- and trans-HDN14–CHO, and D2N14–CHO. The energy levels (above the zero point) deduced from these measurements show a very large isotope shift, and are interpreted as being due to the first excited state of the NH2 wagging frequency. A pyramidal model for formamide will have two equilibrium configurations separated by a potential barrier. With a Manning type potential, a barrier of 370±50 cm−1, hindering the ``inversion-wagging'' type of motion, is determined. The equilibrium value of the normal coordinate calculated from the Manning potential is found to be in good agreement with that found in the structure determination.

High-resolution infrared spectroscopy of formamide and deuterated formamide in a molecular beam

“…We know the perturbation is in the excited state and not the ground state since the microwave spectrum of the ground state can be fit using a rigid rotor 4 or, if greater precision is desired, a rigid rotor with centrifugal distortion constants 8 . The description of the formamide spectrum so far is consistent with the bright state coupling to a single dark state.…”

The extended spectroscopic database for deuterated species of formamide up to 1 THz

“…All the datasets were fitted within the experimental accuracy as it follows from the values of weighted rms deviation we obtained. The fitted datasets include newly measured lines, as well as the measurements reported in earlier studies (Kurland & Wilson 1957;Costain & Dowling 1960). The measurement accuracy of the old studies has not been reported, but we estimate it to be about 0.1 MHz since almost all the residuals of the fit for the lines from Kurland & Wilson (1957) and Costain & Dowling (1960) are smaller than 0.1 MHz.…”

“…To simplify the initial assignment, the first frequency predictions of each deuterated species were calculated using the corresponding rotational rigid rotor constants A, B, C from (Costain & Dowling 1960) and the quartic centrifugal distortion constants from parent species (Kryvda et al 2009). Such a combination of parameters was good enough to assign low-J a R 0,1 transitions in 150-200 GHz frequency range.…”

“…Only a few lines of different NH 2 deuterated species have been observed by Kurland & Wilson (1957) in the frequency range 18-40 GHz. The microwave spectra of ten isotopic species of formamide have been investigated by Costain & Dowling (1960). They measured no more than ten lines for each deuterated species in the frequency range 9-43 GHz.…”

Rotational spectrum of formamide up to 1 THz and first ISM detection of itsν12vibrational state

“…Formamide is a light molecule whose maximum absorption (emission) lies at about 650 GHz below 300 K and an intense spectrum that extends far beyond 1 THz. Several groups have studied the rotational spectrum of formamide since 1955 (Kurland 1955;Kurland & Bright Wilson 1957;Costain & Dowling 1960), but all these studies were quite limited in terms of frequency range. The first revision of the results made by Johnson et al (1972) provided accurate predictions of transition frequencies of the ground state of formamide up to 180 GHz.…”