Rotational spectra of quinoline and of isoquinoline: spectroscopic constants and electric dipole moments q Z. Kisiel, a, * O. Desyatnyk, a L. Pszcz olkowski, a S.B. Charnley, b and P. Ehrenfreund c a Institute of Physics, Polish Academy of Sciences, Al. Lotnik ow 32/46, 02-668 Warszawa, Poland b Planetary Systems Branch, Space Science Division, MS 245-3, NASA Ames Research Center, Moffett Field, CA 94035, USA c Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, Netherlands Received 30 July 2002 Abstract Rotational spectra of quinoline and of isoquinoline have been observed in the centimeter- and millimeter-wave regions. The spectra were assigned on the basis of bands formed by high-J transitions, which were measured up to J 00 6 128 and m 6 234 GHz. Complementary measurements were also made on low-J , centimeter-wave spectra observed in supersonic expansion and with fully resolvednuclearquadrupolehyperfinestructure.Accuraterotational,centrifugaldistortionandhyperfinesplittingconstantsforthe ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are l a ¼ 0.14355(19), l b ¼ 2.0146(17), l tot ¼ 2.0197(17) D for quinoline, and l a ¼ 2.3602(21), l b ¼ 0.9051(14), l tot ¼ 2.5278(20) D for isoquinoline. The experimental observables were found to be rather accurately predicted by MP2/6-31G** ab initio calculations, and corresponding molecular geometries are also reported. Ó 2002 Elsevier Science (USA). All rights reserved. 1. Introduction Quinoline and isoquinoline (C 9 H 7 N, Fig. 1) are among the simplest two-ring heteroaromatic hydrocar- bon molecules. They are of considerable industrial, as well as of potential environmental relevance. Although the vibrational spectra of the two molecules have been analysedquitesometimeago[1],theyhavenotyetbeen investigated by rotational spectroscopy. There are grounds to expect that these molecules may be of as- trophysical importance and the knowledge of the rota- tionalspectrumformsthebasisfortheirdetectioninthe interstellar medium. The list of molecules detected in interstellar space [2] has for a long time been topped by the linear cyanop- olyacetylene HC 11 N. This list is rather deficient in ring molecules, and extension of their number is an enticing goal. Among such molecules are many precursors or building blocks of biologically-important molecules, e.g., amino acids for proteins, purines, and pyrimidines for RNA and DNA bases. The polycyclic aromatic hy- drocarbon (PAH) molecules are believed to be the most abundant free organic molecules in space [3,4], and feature prominently in attempts to solve the longest standing spectroscopic problem of the Diffuse Inter- stellar Bands (DIBs) [5]. However, although there is strong circumstantial evidence for the existence of in- terstellar and circumstellar PAHs through their char- acteristicinfraredemissionbands,astronomicalsearches to positively identify specific aromatic rings have been hampered by their lack of a permanent dipole moment. On the other hand aromatic molecules with nitrogen atoms substituted within the rings have large dipole moments. There is growing evidence that such N-het- erocycles could be an important component of the in- terstellarPAHpopulation[6];theyarealsoknowntobe present in meteoritic material (e.g., benzoquinoline, [7]). The potential for N-heterocycle formation lies in the possibility that other multiply bonded molecules could also take part in the circumstellar chemistry of PAH formation. In particular, as hydrogen cyanide is abun- dant in carbon star envelopes, an HCN addition in the Journal of Molecular Spectroscopy 217 (2003) 115–122 www.elsevier.com/locate/jms q SupplementarydataforthisarticleareavailableonScienceDirect. * Corresponding author. Fax: +48-22-8430926. E-mail address: kisiel@ifpan.edu.pl (Z. Kisiel). 0022-2852/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII:S0022-2852(02)00020-6