Rotational spectra of quinoline and of isoquinoline: spectroscopic constants and electric dipole moments

Journal of Molecular Spectroscopy 217 (2003) 115–122 www.elsevier.com/locate/jms

Rotational spectra of quinoline and of isoquinoline: spectroscopic constants and electric dipole momentsq Z. Kisiel,a,* O. Desyatnyk,a L. Pszcz ołkowski,a S.B. Charnley,b and P. Ehrenfreundc b

a Institute of Physics, Polish Academy of Sciences, Al. Lotnik ow 32/46, 02-668 Warszawa, Poland 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 resolved nuclear quadrupole hyperfine structure. Accurate rotational, centrifugal distortion and hyperfine splitting constants for the ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are la ¼ 0.14355(19), lb ¼ 2.0146(17), ltot ¼ 2.0197(17) D for quinoline, and la ¼ 2.3602(21), lb ¼ 0.9051(14), ltot 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 (C9 H7 N, Fig. 1) are among the simplest two-ring heteroaromatic hydrocarbon molecules. They are of considerable industrial, as well as of potential environmental relevance. Although the vibrational spectra of the two molecules have been analysed quite some time ago [1], they have not yet been investigated by rotational spectroscopy. There are grounds to expect that these molecules may be of astrophysical importance and the knowledge of the rotational spectrum forms the basis for their detection in the interstellar medium. The list of molecules detected in interstellar space [2] has for a long time been topped by the linear cyanopolyacetylene HC11 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, q

Supplementary data for this article are available on ScienceDirect. Corresponding author. Fax: +48-22-8430926. E-mail address: [email protected] (Z. Kisiel). *

e.g., amino acids for proteins, purines, and pyrimidines for RNA and DNA bases. The polycyclic aromatic hydrocarbon (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 Interstellar Bands (DIBs) [5]. However, although there is strong circumstantial evidence for the existence of interstellar and circumstellar PAHs through their characteristic infrared emission bands, astronomical searches 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-heterocycles could be an important component of the interstellar PAH population [6]; they are also known to be 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 abundant in carbon star envelopes, an HCN addition in the

0022-2852/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S 0 0 2 2 - 2 8 5 2 ( 0 2 ) 0 0 0 2 0 - 6

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(a)

(b)

Fig. 1. The geometries of (a) quinoline, and (b) isoquinoline. The orientations of inertial axes and of electric dipole moments are indicated.

sequence that leads to benzene can instead lead to pyridine. Acetylene polymerization of pyridine, or HCN addition to the phenyl radical can likewise lead to the nitrogen-substituted analogues of napthalene: quinoline and isoquinoline [8]. Although early radioastronomical searches for organic rings obtained negative results [9], more recently the small 3-member azaheterocycles 2H-azirine and aziridine have been detected in interstellar clouds [10]. Astronomical searches for the single-ring aromatics pyridine and pyrimidine have also recently been carried out. Pyrimidine is not detected in interstellar clouds [11] but both molecules are tentatively detected in carbon star envelopes. The present paper reports the results of a comprehensive analysis of the rotational spectra of quinoline and of isoquinoline, with the objective of deriving new molecular information and in order to prepare the ground for teledetection of the two molecules.

2. Experimental Rotational spectra were recorded on commercially available samples (Merck), by using the two different spectrometers available in Warsaw. Millimetre-wave (MMW) spectra were measured at room-temperature and at sample pressure of 15–30 mTorr by using the mm-wave BWO-based, source modulation spectrometer described in [12,13]. Frequency control in this spectrometer consists of two phase-lock loops (PLL) driven from a 3 GHz synthesiser, and utilising a 18–26.5 GHz K-band transfer oscillator. The klystrons used for this purpose have recently been upgraded to a low cost, and much more convenient 2–18.6 GHz solid-state oscillator source (HP-89290B/HP-8620C). This oscillator is locked to the 5th harmonic of the synthesiser, and its frequency stabilised output is then doubled in frequency with an active doubler (Narda DBS-1826X220) for use in the higher PLL loop locking the BWO source. The use of coaxial waveguides and presetting the transfer oscillator via the IEEE-488 interface allows unattended operation of the lower PLL loop.

Low frequency rotational spectra were measured in supersonic expansion using a Flygare-type cavity, Fourier-transform microwave (FTMW) spectrometer [14]. The samples were held in small stainless-steel test tubes placed close to the pulsed expansion nozzle, and in the path of Ar carrier gas supplied at a backing pressure of ca. 1 atm. Both samples had to be heated to 50–70 °C in order to generate sufficient vapour pressure for observation of spectra. Stark measurements were made with a set of parallel plate electrodes designed to produce uniform electric field at the large electrode separation that does not perturb the cavity mode of the FTMW spectrometer [15,16]. Electric field calibrations were made on the basis of the J ¼ 1 0 transitions of CH3 CN, l ¼ 3:92197 ð13Þ D [17], and of CH3 I, l ¼ 1:6406ð4Þ D [18]. Stark measurements for molecules with a quadrupolar atom were fitted with program QSTARK described in [15] and available from the PROSPE database [19].

3. Rotational spectrum In the absence of pertinent prior information we predicted rotational spectra from ab initio rotational constants and dipole moments calculated with PC-GAMESS [20,21] at the HF/6-31G** and MP2/6-31G** levels. The calculations established that the electric dipole moment is oriented rather differently in the two molecules, see Fig. 1, leading to rather different rotational spectra. Quinoline would be characterised by predominantly lb spectra, whereas isoquinoline should exhibit mainly la spectra. At MMW frequencies the rotational spectra were expected to be dominated by high-J , R-type bands characteristic of planar molecules [22–24]. In the nomenclature of [23] these are n ¼ 2, type-IIþ bands arising from the ratio A þ B ¼ nð2CÞ between rotational constants, which is imposed by the planarity condition. Since such bands can be formed from both a R- and b R-type transitions, they would be expected to feature prominently in rotational spectra of both quinoline and isoquinoline. In the room temperature spectrum the high-J bands were predicted to peak in intensity at frequencies near 200 GHz, and this is where measurements were commenced. The relevant bands were in fact readily located in the video mode of the MMW spectrometer, and their analysis facilitated rapid assignment of the rotational spectra. Sample MMW spectra of quinoline and isoquinoline are reproduced in Figs. 2 and 3, and illustrate the most relevant features of rotational spectra of the two molecules. The distribution of lines in the n ¼ 2 type-IIþ bandheads is primarily sensitive to values of the constants C, DJ , and dJ in the rotational Hamiltonian, and various perturbations relative to a pattern of uniformly increasing line spacing are possible. These may range

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Fig. 2. Example of a ground-state n ¼ 2, type-IIþ , high-J band in the mm-wave rotational spectrum of quinoline. The band consists of degenerate b R1;1 , b R1;1 transitions with successively decreasing values of J and increasing values of K1 . The bandhead doubles back on itself and its first line 00 consists of overlapped 1151;115 1140;114 , 1150;115 1141;114 transitions. The values of K1 for the b R1;1 component are indicated in the inset. Some b b lower-J R-type lines and several Q-type lines are also visible.

Fig. 3. Example of a ground-state n ¼ 2, type-IIþ high-J band in the mm-wave rotational spectrum of isoquinoline. In this case the band is formed from overlapped a R0;1 transitions, so that the first line consists of degenerate 1190;119 1180;118 , 1191;119 1181;118 transitions. A weaker n ¼ 3, typeII band formed by b R transitions is also visible. Values of J 00 are marked for both bands. Several high K, a R lines, corresponding to tails of the familiar type-I a R bands are also present, in particular the diffuse sequence for J 00 ¼ 98.

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from a local tightening in the pattern of lines to considerable folding of the bandhead on itself—both of these effects were observed in the spectrum of pyrimidine [25]. For quinoline and isoquinoline it turns out that only the first few lines of the band are folded back, as visible in the higher resolution inset in Fig. 2. A total of ca. 250 MMW rotational transitions were measured for each molecule, over the 164–234 GHz frequency region. For quinoline the lines from type-IIþ bandheads were augmented by measurements on transitions that do not fall into such easily discernible patterns. These are lower-J b R-type transitions and b Q-type transitions, some of which are marked in Fig. 2. The second nonzero dipole moment component, la , is so small that no la -type transitions were observed. In the case of isoquinoline the smaller, lb dipole moment component, is sufficiently large for observation also of b R-type transitions. Fig. 3 contains an example of an n ¼ 3, type-II band formed by such transitions. This type of band is formed by condensation of transitions due to the properties of energy levels at the prolate asymmetric top limit, and will occur if values of rotational constants approach the ratio ð2AÞ ¼ nðB þ CÞ [23]. In addition to the transitions belonging to the two clear band types the MMW ground-state spectrum of isoquinoline also contains many high-K transitions. These can be regarded as tails of the familiar type-I a R0;1 -bands, consisting of pileups of lines with various values of K1 for a given J [23]. Complete lists of measured rotational transitions are given in Tables S1 and S2 of the supplementary information for this paper, in which the MMW measurements have been combined with quadrupole-removed frequencies for low-J transitions measured in supersonic expansion (see further below). Spectroscopic constants obtained on fitting WatsonÕs A-reduced asymmetric-rotor Hamiltonian in representation Ir [26] are given in Table 1. Even though the coverage of quantum number values in the data is quite extensive, it was not necessary to invoke any of the sextic distortion constants. The inertial defects for both molecules are small and negative, and can be 2 for the slightly compared with Di ¼ 0:1113ð12Þ u A lighter two-ring indole molecule [27]. These values are rather different from positive, and smaller magnitude values for single ring aromatics, such as Di ¼ 0:03496ð2Þ u 2 for pyrimidine [25]. To answer the question of whether A the small negative values carry implications of non-planarity in the double-ring molecules would require, first of all, reliable force field calculations. The precise quartic centrifugal distortion constants allow evaluation also of the quartic defect, Dq [28]. The values of Dq for quinoline and isoquinoline are some of the smallest yet determined. These values are much lower than those in the comparison for a series of increasingly more heavy molecules made in [29], and also lower than Dq ¼ 0:0185ð54Þ MHz2 for py-

Table 1 The fitted spectroscopic constants for quinoline and isoquinoline in WatsonÕs A-reduced, Ir Hamiltonian Quinoline

Isoquinoline a

A (MHz) B (MHz) C (MHz)

3145.533013 (79) 1271.577972 (74) 905.739406 (44)

3199.00020(30) 1237.931586 (74) 892.753595 (48)

DJ (Hz) DJK (Hz) DK (Hz) dJ (Hz) dK (Hz)

19.1103 (32) 47.0313 (98) 161.461 (19) 5.6621 (15) 60.622 (24)

18.8548 (29) 47.004 (12) 157.20 (24) 5.4543 (17) 61.465 (37)

2

) Di b (u A Dq c (MHz2 ) Nlines d rfit (kHz) rw

)0.134059 (36) 0.000488 (22) 262 28.9 0.967

)0.137857 (42) 0.000503 (28) 272 25.4 0.850

a

The quantities in parentheses are standard errors in units of the least significant digit of the value of the constant. b Inertial defect, Di ¼ Ic  Ia  Ib . c Quartic defect, Dq ¼ 4CDJ  ðB  CÞDJK  2ð2A þ B þ CÞdJ þ 2ðB  CÞdK . d The number of fitted rotational transitions.

rimidine, and Dq ¼ 0:00391ð22Þ MHz2 for trichloroethylene [23]. The present values thus extend the range of the known general inverse scaling of this quantity with the magnitude of the moments of inertia. 4. Nuclear quadrupole splitting constants Assignment of the high-J MMW spectra allowed measurement also of the lowest J -transitions at subDoppler resolution of supersonic expansion, and determination of nuclear quadrupole splitting constants from completely resolved hyperfine structure due to the 14 N nucleus. More than 50 different hyperfine components were measured for each of the two molecules, with the lowest-J transitions being 212 101 at 5.8 GHz for quinoline and 322 221 at 6.4 GHz for isoquinoline. The observed and the obs.)calc. frequencies are summarised in Tables S3 and S4 of the supplementary material and the derived constants are reported in Table 2. The fits were performed with PickettÕs program SPFIT [30] and all constants in the rotational part of the Hamiltonian were fixed at the values in Table 1. The deviations of fit are well within the nominal 2 kHz accuracy of the FTMW spectrometer and the diagonal constants in the inertial nuclear quadrupole splitting tensor are precisely determined. Regrettably, determination of the single non-zero off-diagonal constant vab proved elusive, due to the absence of any local energy level perturbations. The existence of such perturbations delivered precise vab for the nitrogen atom in 2-chloroacrylonitrile [31], whereas vab for quinoline could not be determined with significance, and is only poorly deter-

Z. Kisiel et al. / Journal of Molecular Spectroscopy 217 (2003) 115–122 Table 2 The fitted and the calculated nuclear quadrupole splitting constants for the

14

N nucleus in quinoline and isoquinoline

Quinoline

vaa (MHz) vbb (MHz) vcc (MHz) vab (MHz) rfit (kHz) Ntrans c vzz (MHz) hza (deg) hstr d (deg)

119

Isoquinoline

obs.

calc.a

obs.

calc.a

1.4629 (10) )4.6841 (11) 3.2213 (11) [0.35]b 1.31 17.54

1.69 )4.84 3.14 0.35

)3.5114 (15) 0.1304 (19) 3.3810 (19) 2.81 (42) 1.45 19.58 )5.04 (25) 28.5 (20)

)3.51 0.11 3.39 2.87

)4.86 93.17 90.34

)5.09 28.26 29.89

a

From MP2/6-31G** calculation scaled by factor of 1.125, which was determined from experimental results for pyrimidine, [25]. Assumed value. c The number of measured rotational transitions, and the total number of fitted hyperfine components, respectively. d The angle between the \ðCNC) bisector and the right-hand side of the a-axis in Fig. 1. b

mined for isoquinoline. The experimental values for the v tensor components are found to be satisfactorily reproduced by appropriately scaled ab initio field gradient calculations, Table 2. For quinoline and isoquinoline the direction of the z-axis of the principal nuclear quadrupole tensor is also calculated to be within several degrees of the \(CNC) bisector.

5. Electric dipole moments The results of Stark measurements on selected rotational transitions of quinoline and isoquinoline are

(a)

summarised in Fig. 4 and in Table 3. The choice of rotational transitions was guided by the constraints of sufficient intensity of separated components and of appreciable Stark shifts at the relatively low fields available in the parallel plate, cavity FTMW experiment. The dipole moments of both molecules are in excess of 2 D so that for the two selected transitions electric fields of less than 220 V/cm were sufficient. The Stark electrodes were separated by 26.93 cm, so the applied fields were produced by application of potential differences of less than 6 kV across the electrodes. The hyperfine splitting from the 14 N nucleus is similar in magnitude to the Stark splitting, so that the two cannot be treated separately.

(b)

Fig. 4. Stark effects in (a) the 220 111 rotational transition of quinoline, and (b) the 523 422 rotational transition of isoquinoline. Circles mark experimental measurements and the curves are calculated from the fitted dipole moments. The label next to each Stark component denotes the pertinent value of MF .

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Table 3 The fitted and the calculated electric dipole moments for quinoline and isoquinoline Quinoline

la (D) lb (D) ltot (D) rfit (kHz) Nb a b

Isoquinoline

obs.

calc.a

obs.

calc.a

0.14355 (19) 2.0146 (17) 2.0197 (17) 2.21 67

0.216 2.012 2.023

2.3602 (21) 0.9051 (14) 2.5277 (20) 2.12 51

2.456 1.018 2.658

Calculated at the MP2/6-31G** level. The number of fitted data points.

This is confirmed by Fig. 4, in which only a few of the measured Stark components exhibit pure second order behaviour of frequency shift with the applied electric field. Various departures from second order behaviour are apparent, and the use of the intermediate field Stark analysis [32,15] as built into program QSTARK was mandatory. The constants in the rotational and in the nuclear quadrupole parts of the Hamiltonian were fixed at values in Tables 1 and 2, so that only the two nonzero dipole moment components, la and lb , were the parameters of fit. The observed and the obs.)calc. frequencies for the measured Stark components are reported in supplementary Tables S5 and S6, and the resulting dipole moments are given in Table 3. The deviations of fit are again close to the frequency accuracy of the spectrometer and lead to nominal errors in the dipole moment components which are several times lower than those in Table 3. However, we choose to report more realistic error estimates, by following the previous practice of

increasing the uncertainty in the electric field calibration [16] in an attempt to account for many small effects outside direct experimental control. These include possible systematic changes in lineshape with applied electric field, frequency dependence of the matching between the gas expansion plume, the microwave cavity mode and the electric field, and the possibility of various polarisation effects. Although the very small la component for quinoline is determined very precisely, its relative precision is still comparable to that of the lb component. The ab initio values for the dipole moments are found to compare quite well with experiment, especially in view of the use of only a relatively moderate level of computation.

6. Molecular geometries The ab initio rotational constants calculated at the MP2/6-31G** level are A ¼ 3140, B ¼ 1271, C ¼ 905

Fig. 5. The calculated, MP2/6-31G**, geometries of quinoline (left), and isoquinoline (right). The three principal resonance structures are drawn for each molecule.

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MHz for quinoline, and A ¼ 3197, B ¼ 1237, C ¼ 892 MHz for isoquinoline. These values are in remarkably good agreement with the experimental rotational constants in Table 1, implying that the calculated geometries may be a fair reflection of the actual geometry for each molecule. They are certainly expected to be an advance on the geometries assumed in [1], and for this reason the presently calculated geometries are summarised in Fig. 5. Introduction of the nitrogen atom leads to significant perturbations in the heterosubstituted ring, although the angular geometry of the lefthand ring is seen to be relatively unperturbed. The differences in the lengths of CC bonds are similar to those in naphtalene, for which they are successfully rationalised by averaging the three principal canonical structures [33]. This is also the case presently. For quinoline, for example, the shorter C5 C6 , C7 C8 , C3 C4 bonds of ca.  are all bonds which are double-bonds in two of 1.38 A the three resonance structures, while the longer C9 C10 , C5 C10 , C6 C7 , C8 C9 , C4 C10 , and C2 C3 bonds are double bonds only in one out of the three structures. Similar arguments readily rationalise the relative lengths of the two CN bonds in each molecule.

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dipole moments were determined, and were found to be well predicted by MP2/6-31G** ab initio calculations. The rotational spectra of the two molecules have been characterised in sufficient detail for the requirements of radioastronomy. The different orientations of the dipole moments in quinoline and in isoquinoline lead to prediction of rather different low temperature rotational spectra. The high-J bands will be considerably attenuated on cooling and the maximum in the absorption profile will be defined by lower-J transitions. At 50 K, for example, the maximum intensity for quinoline is predicted to be at 143 GHz and will be due to b R-transitions. For isoquinoline the maximum intensity will be almost 30% lower and will be at 76 GHz (a R-transitions). In addition, according to the MP2/6-31G** energies, quinoline is more stable than isoquinoline by about 5 kJ mol1 . Thus on a simple equilibrium basis, without taking into account the astrophysical formation mechanisms and possible non-LTE conditions, it is quinoline that would be a more attractive target for searches than isoquinoline.

Acknowledgments 7. Vibrational satellites The MMW rotational spectra in Figs. 2 and 3 contain a considerable number of unassigned lines, which belong to rotational transitions in low lying vibrationally excited states. It is known from the vibrational spectrum [1] that the lowest frequency in-plane and out-of-plane modes for both molecules are near 200 and 180 cm1 , respectively. The next higher fundamental is at ca. 380 cm1 , so that the v ¼ 1 satellites for the two lowest modes should be readily visible. This is probably the case, as for isoquinoline the prominent groups of lines to high frequency of 213.41 and of 213.49 GHz (the latter just outside the coverage of Fig. 3), were tentatively assignable to n ¼ 2, type-IIþ band sequences. Nevertheless, effective single state fits to the assigned transitions revealed the need for values of the DJ constant, which differed by more than a factor of 2 from its value in the ground state. This is indicative of rather strong Coriolis coupling between the lowest vibrational states. The interaction is probably even stronger in quinoline, for which even a tentative satellite assignment was not possible.

8. Discussion The ground-state rotational spectra of quinoline and isoquinoline were successfully assigned in top–down manner, from high-J bands in broad-band MMW spectra. Accurate spectroscopic constants and electric

Financial support from the Institute of Physics and from the State Committee for Scientific Research, grant KBN-3T09A-126-17 is gratefully acknowledged. Theoretical astrochemistry at NASA Ames is supported by NASAÕs Origins of Solar Systems and Exobiology Programs through NASA Ames Interchange NCC2-1162. Partial support from The Netherlands Research School for Astronomy (NOVA) is also acknowledged.

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