Confirmation of the Electric Field Gradients at the Nitrogen Nuclei in N-Cyanomethanimine, CH₂NCN, from Low-J Rotational Transitions

Abstract

The results of a high resolution microwave study of low-J rotational transitions of N-cyanomethanimine are reported. The 14N quadrupole coupling constants fitted to the observed hyper fine splittings are: xaa (imine) = 2.032(9) MHz, / fcfc(imine) = -4.586(7) MHz, and / aa(nitrile) = -3.254(8) MHz, / fcfc(nitrile) = 0.844(16) MHz. Knowledge of these constants together with the rotational constants and centrifugal distortion constants determined earlier provides the basis for accurate prediction of hitherto unmeasured rotational transitions for radioastronomy. Introduction ExperimentalBecause of its astrophysical interest, the rotational spectrum of N-cyanomethanimine has been studied extensively by and by Winnewisser et al. at Giessen [3,4]. The latter group has also successfully resolved and analysed the 14N hyperfine structure in the millimeter wave spectrum. The hfs structure is a valuable tool for fingerprint line identification in radioastronomy. However two sets of quadrupole coupling constants were compatible with the observed mm wave spectra, and the authors based the choice of the correct set on the comparison with ab initio field gradients. Now, it turned out recently that present day ab initio methods may still have problems with the calculation of field gradients, especially at a sp2 hybridized 14N nucleus. We therefore initiated a complementary study of the 14N hyperfine patterns of low-J rotational transitions. These low-J hfs patterns differ sufficiently for the above two sets of coupling constants. Thus an unambiguous experimental deter mination of the quadrupole coupling constants and the reliable prediction of the hfs patterns also of hith erto unmeasured rotational transitions becomes pos sible. Since N-cyanomethanimine is highly unstable in the absorption cell (polymerisation), a flow system was set up. The sample was produced immediately before the front end of the cell by flash pyrolysis of trimethylenetetrazole [6] in a quartz tube.Our quartz tube had an inner diameter of 1 cm. The length of the heated zone was 20 cm. In order to min imize wall collisions the pyrolysis products were fed into the cell coaxially.Typical total pressures at the front end of the 1.8 m X-band absorption cell were around 10 mTorr and at the rear end around 1 to 3 mTorr. Aluminum wave guide cells proved to be superior to our standard brass waveguide cells. Preconditioning of the system with CF3COOC2H 5 (as used by Bak and Svanholt in their preparation of the molecule from dimethylcyanamide [2]) improved the signal to noise ratio in the spectra at least by a factor of two.To record the spectrum, a micro-computer con trolled cw-microwave spectrometer with 30 kHz Stark-effect modulation and with phase stabilized backward wave oscillators as radiation sources was 0932-0784 / 89 / 0300-305 S 01.30/0. -Please order a reprint rather than making your own copy. used. The computer control system, which also pro vides averaging and data handling capabilities, is de scribed in detail in [7], Typical experimental linewidths we...

First Detection in Space of the High-energy Isomer of Cyanomethanimine: H2CNCN

“…N-cyanomethanimine (H 2 CNCN) is a planar asymmetric-top molecule with a large total dipole moment of 4.84D (Bak et al 1978;Bak & Svanholt 1980). To perform its search toward G+0.693, we used the spectroscopic entry 054514 (2014 February) from the CDMS catalog, which incorporates the rotational spectroscopy from the Bak & Svanholt (1980), Winnewisser et al (1984), andStolze et al (1989) experimental works. We have identified a plethora of distinct a-type rotational transitions belonging to this species, which cover an energy level range between J up = 3 and J up = 11 and correspond to different K a = 0, 1, 2 ladders.…”