The very low methyl group V3 barrier of cis N-methylformamide: A–E doubling from the free jet rotational spectrum☆
Introduction
N-Methylformamide (NMF) is of some relevance as anti-carcinogenic agent [1]. The study of its configuration, structure, and large amplitude motions can be of help in understanding the mechanism of this effect. For this reason NMF has been the subject of extensive spectroscopic [2], [3], [4], [5], [6] and electron diffraction investigations [7]. The stability of the peptide-like frame (–CO–NH–) reduces the number of stable conformers to two: the cis and trans forms, due to the position of the methyl group with respect to the aldehydic oxygen (see Fig. 1). The very low barrier to the methyl group internal rotation (V3≈700 J/mol) prevented the assignment of its rotational spectrum until recently, when, according to some ab initio calculations, the spectrum of the A lines of the cis conformer has been assigned [8]. The A–E splittings due to the V3 barrier hindering the internal rotation of the methyl group were estimated to be very large, and since the E-type lines do not follow a regular pattern, it was not possible to locate them in the very dense room temperature spectrum. The V3 barrier was estimated from the ‘pseudo’ inertial defect [9], [10]. Here we report the identification of several E lines in the simplified jet cooled spectrum, and confirm, on the basis of a fit of the A–E splittings, a very low value of the V3 barrier. A recently developed model [11] for the methyl internal rotation was, however, required in order to obtain an accuracy of the fit compatible with measurement precision of the rotational spectra.
Section snippets
Experimental
A sample of NMF was purchased from Aldrich and used without further purifications. The details of the free jet spectrometer are described elsewhere [12], [13]. In the experiment the sample seeded in argon at a stagnation pressure of ca. 200 Pa at room temperature was expanded adiabatically to about 50 mPa through a 0.35 mm diameter nozzle reaching an estimated ‘rotational’ temperature of about 10 K. The high sweep rate, the natural line shape and intensities, and the possibility of playing with the
Combined analysis of ‘A’ and ‘E’-type lines
The positions of the E-type lines were calculated from the inertial defect of the A-type spectrum [8]. A sample of pairs of A–E component lines, showing A–E splittings up to 10 GHz, is given in Table 1. The full set of measured transitions is available from the authors. Table 2 shows the spectroscopic constants obtained from several different fits. In the first column the result of only the A type lines is reported. By comparison with the results of the adjacent columns one can see the enormous
Conclusions
The low value for the V3 barrier to internal rotation (660±10 J/mol) has been precisely obtained from the A–E line splittings measured in a free jet experiment. The E components, away up to several GHz from the twin A component lines, have been easily identified with the millimeter wave free jet absorption technique. The spectrum was indeed very much simplified with respect to the room temperature one. To assign the E lines we had the advantage, with respect to molecular beam Fourier Transform
Acknowledgements
W.C. thanks the University of Bologna, the Ministero dell'Università e della Ricerca Scientifica e Tecnologica, and the Consiglio Nazionale delle Ricerche for financial support, and Mr A. Millemaggi for technical help.
References (16)
J. Mol. Spectrosc.
(1958)J. Mol. Spectrosc.
(1960)- et al.
Spectrochim. Acta
(1956) - et al.
J. Mol. Struct.
(1995) - et al.
Chem. Phys. Lett.
(1996) - et al.
J. Mol. Spectrosc.
(1987) - et al.
Cancer Res.
(1979) J. Chem. Phys.
(1955)
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This paper is dedicated to Professor Paolo G. Favero and Professor Helmut Dreizler in appreciation of their significant contributions to the field of microwave spectroscopy.
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Member of the Carrera del Investigador Cientifico y Tecnologico, CONICET, Rep. Agentina.