Fourier transform microwave spectrum of pyridine–neon

Journal of Molecular Spectroscopy 251 (2008) 176–179

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Fourier transform microwave spectrum of pyridine–neon Biagio Velino a, Walther Caminati b,* a b

Dipartimento di Chimica Fisica e Inorganica dell’Università, Viale Risorgimento 4, I-40136 Bologna, Italy Dipartimento di Chimica ‘‘G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 30 January 2008 In revised form 19 February 2008 Available online 29 February 2008 Keywords: Rotational spectroscopy Molecular complexes Large amplitude motions Free jets Neon

a b s t r a c t The rotational spectra of normal, and various 15N, 13C and 22Ne species of pyridine–neon have been investigated by molecular beam Fourier transform microwave spectroscopy. The obtained results allowed for the experimental determination of the structure and of the 14N quadrupole coupling constants of the complex. The spectroscopic parameters are compared to those of other rare gases–pyridine complexes. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Several complexes of six-membered aromatic molecules with rare gases (RG) have been investigated by rotational spectroscopy. The rotational spectra of benzene–RG, with RG = Ne [1,2], Ar [2,3], Kr [4] and Xe [2] have been observed by Fourier transform microwave spectroscopy (FTMW) combined with pulsed jet techniques. Also in the case of pyridine, the rotational spectra of four pyridine– RG adducts have been measured, with FTMW spectroscopy for RG = He [5], Ar [6,7] and Kr [6], and with free jets mmw spectroscopy for RG = Ne [8] and Ar [9]. In addition, the free jets mmw spectra of pyrimidine–Ne [10], pyridazine–Ne [11] pyrimidine–Ar [12] and pyridazine–Ar [13] have been reported. In all these cases, the RG atom is located on one side of the aromatic ring, and the barriers in going from above to below the aromatic planes are high enough to prevent tunnelling effects to be observed, except for some small splittings in the case of pyridine–He. Information on the van der Waals motions has been obtained. Generally the RG stretching motion has been considered isolated from the bending motions, and its frequency has been obtained from the DJ centrifugal distortion constant within a pseudo-diatomic approximation. From these frequencies, approximate values of dissociation energies have been obtained [8–13]. Also information on the bending motions has been obtained, either from centrifugal distortion constants [14], or from mass distribution and Coriolis effects [12]. It has been claimed that the p-electron systems of aromatic molecules play important roles in non-covalent interactions and * Corresponding author. Fax: +39 051 2099456. E-mail address: [email protected] (W. Caminati). 0022-2852/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2008.02.017

that in rare gas atoms containing van der Waals systems, for example, the aromatic p-electrons provide a significant driving force for the weak intermolecular bond [5,15]. However, distributed polarizability models, based on contribution from dispersion interactions, the magnitude of which depends on the polarizability of the involved atoms or molecules, have been found very effective in finding van der Waals potential energy surfaces [16,17]. Here we report the FTMW spectra of several isotopologues of pyridine  Ne (from now on Pyr  Ne), and discuss the obtained results, that is the quadrupole coupling constants of 14N and the structure of the complex. The principal axis systems of Pyr   Ne (left) and Pyr (right) are shown in Fig. 1.

2. Experimental part The MB-FTMW spectrum in the 6–18.5 GHz frequency region was measured using a COBRA-type [18] pulsed supersonic jet Fourier transform microwave (FTMW) spectrometer [19] described elsewhere [20], recently updated with the FTMW++ set of programs [21]. Neon was passed over pyridine at room temperature, and the mixture was expanded from ca 3 bar to about 105 mbar. Each rotational line is split by the Doppler effect, enhanced by the molecular beam expansion in the coaxial arrangement of the supersonic jet and resonator axes. The rest frequency is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of frequency measurements is better than 3 kHz and lines separated by more than 7 kHz are resolvable.

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Table 1 Experimental transition frequencies (MHz) of Pyr14N  Ne and one related isotopologue

c=z

a=z

J0 ðK 0a ; K 0c Þ

Ne

J00 ðK 00a ; K 00c Þ

F 00

2(1, 2)–1(0, 1)

3 2 1 2 1

2 1 0 2 1

2(2, 1)–1(1, 0)

3 2 1

2 1 0

3(1, 3)–2(0, 2)

4 3 2 3 2

3 2 1 3 2

12289.8643 12288.6157 12290.5122 12289.6444 12288.9079

4(0, 4)–3(1, 3)

5 4 3

4 3 2

13840.8422 13842.0917 13840.4999

4(1, 4)–3(0, 3)

5 4 3

4 3 2

15984.4745 15983.3110 15984.8697

c=y b=y

r

θ N

N

b=x

PYRIDINE-Ne

a=x PYRIDINE

Fig. 1. Sketches of Pyr  Ne and Pyr showing the permutation of the principal axes in going from the isolated molecule to the complex. R and h are the determined van der Waals parameters.

15 N-pyridine (99%) was obtained from Aldrich, and used without further purifications.

20

F0

Ne

8584.6473 8583.4040 8586.5724 8584.3823 8584.1285

22

Ne

8310.4598 8309.0130 8312.1906 8309.9938 8309.7361 10798.1044 10796.6183 10799.3392 11833.8722 11832.6195 11834.5230 11833.6544 11832.9047

15347.6154 15346.4457 15348.0144

3. Rotational spectrum where HR, HCD and HQ are the rotational, centrifugal distortion and 14 N quadrupole (when appropriate) contributes to the total Hamiltonian. Since Pyr  Ne is a prolate near-symmetric top, the S-reduction and the Ir-representation have been chosen [23]. The same Hamiltonian has been used for the 22Ne species, while for the 15N-isotopologues, the HQ component of the Hamiltonian has been omitted. All obtained spectroscopic constants are given in Table 3.

From the rotational constants of Ref. [8] it was easy to locate the rotational transitions of the normal species of Pyr  Ne, falling in the 8–18 GHz frequency region of our FTMW spectrometer. Later on, the rotational spectra were assigned also for the additional six isotopologues with 22Ne, 15N, 15N–13C(2), 15N–13C(3), 15 N–13C(4) and 15N–22Ne isotopic substitutions. To measure the 13 C species in natural abundance, the 15N-enriched pyridine was used, since their rotational transitions, not split for the quadrupole, are much stronger than the corresponding lines of the 14N (normal) species. Fig. 2 shows the 14N-quadrupole hyperfine component lines of the 313–202 transition of the normal species. The frequencies of all measured lines are listed in Tables 1 and 2. The newly measured frequencies of the normal species, together with those previously measured in the millimeter wave range [8], for a total of 57 rotational lines, have been fitted with SPFIT Pickett program [22], according to the Hamiltonian: H ¼ HR þ HCD þ HQ

4. Structure When choosing Pyr-15N  Ne as parent molecule, it is possible to obtain, from the available experimental data, the rs coordinates [24] of all heavy atoms of the complex in the principal axes system of Pyr-15N  Ne. They are reported in Table 4. It appears clear that the jcj-coordinates of the N, C4 and Ne atoms, which should be zero by symmetry, have small imaginary value, probably because of the large amplitude van der Waals vibrations. They prove unambiguously that the Ne atom is considerably shifted towards the N

313←202

2←2

3←3

2←1

3←2

4←3

ð1Þ

12288.4 Fig. 2. The five F

0

F

00 14

12288.9

12289.4

12289.9

12290.4

12290.9

N quadrupole component lines of the 313–202 transition of normal Pyr  Ne. Each of them is split into two Doppler components.

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Table 2 Experimental transition frequencies (MHz) of Pyr15N  Ne and related isotopologues J0 ðK 0a ; K 0c Þ

J00 ðK 00a ; K 00c Þ

15

2(1, 2)–1(0, 1) 2(2, 1)–1(1, 0) 2(2, 0)–1(1, 1) 3(0, 3)–2(1, 2) 3(1, 3)–2(0, 2) 3(2, 2)–2(1, 1) 3(2, 1)–2(1, 2) 4(0, 4)–3(1, 3) 4(1, 4)–3(0, 3) 5(0, 5)–4(1, 4)

15

N

8504.826 10771.596 10804.244 10080.061 12173.346 14454.523 14553.805 13837.353 15825.470 17602.955

N–13C2

15

N–13C3

15

N–13C4

15

N–22Ne

8454.815 10672.710 10701.696 10049.138 12112.281

8449.642 10667.419 10695.213 10035.979 12105.113

8449.642 10667.419 10695.213 10035.979 12105.113

8233.678 10679.292 10709.031 9436.804 11723.325 14182.116

15754.915

15746.258 17507.271

15746.258 17507.271

13008.514 15197.786 16588.605

Table 3 Spectroscopic constants of all measured isotopologues of Pyr  Ne 22

Normal a

A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz HJ/Hz HJK/Hz HKJ/Hz HK/Hz 3/2 vaa/MHz 1/4(vbb–vcc)/MHz Nc r/kHzd

3011.463 (3) 1876.4048(7) 1858.0647(9) 20.47(3) 90.6(2) 104.9(2) 0.528(3) 0.023(1) 1.56(6) 20.6(5) 62.4(7) 38.3(4) 4.886(6) 1.539(2) 57 1e

15

Ne

N–13C(2)

N

3010.668(7) 1783.576(3) 1766.8662(4) [20.47]b [90.6] [104.9] [0.528] [0.023] [1.56] [20.6] [62.4] [38.3] 4.910(6) 1.542(2) 13 1

2976.505 (4) 1875.096(1) 1843.127(1) 20.01(1) 91.1(1) 106.4(7) 0.56(2) [0.023] [1.56] [20.6] [62.4] [38.3] — — 10 4

a

15

15

N–13C(3)

2945.6728(7) 1865.1653(7) 1836.732(1) 19.75(4) [90.6] [104.9] [0.528] [0.023] [1.56] [20.6] [62.4] [38.3] — — 6 1

2944.3362(7) 1862.7329(7) 1835.4539(6) 19.76(2) [90.6] [104.9] [0.528] [0.023] [1.56] [20.6] [62.4] [38.3] — — 7 3

15

N–13C(4)

15

N–22Ne

2944.0998(6) 1870.4547(7) 1825.9342(8) 19.52(3) [90.6] [104.9] [0.528] [0.023] [1.56] [20.6] [62.4] [38.3] — — 8 6

2975.770(6) 1782.181(2) 1752.967(2) 18.67(2) 85.2(3) 99(2) 0.48(3) [0.023] [1.56] [20.6] [62.4] [38.3] — — 9 3

The most abundant isotopic species is labelled ‘‘normal”. a Errors in parentheses are expressed in unit of the last digit. b The values in square brackets are undetermined in the fit and fixed at the value of the normal species. c Number of transitions in the fit. d Standard deviation of the fit. e For the normal species, also nn millimeter-wave (mmw) transitions from Ref. [8] have been fitted. The standard deviation indicated here is relative to the FTMW transitions. The overall reduced deviation of the fit (r/rexp) is 0.83 when the measure errors on the FTMW and mmw transitions are set to 3 and 100 kHz, respectively.

Table 4 Substitution coordinates (Å) of the heavy atoms in the principal axes system of Pyr15N Ne jaj

Exptl. Calc.a

jbj

Exptl. Calc.

jcj

Exptl. Calc.

N

C2

C3

C4

0.462(3) 0.4247

0.551(3) 0.5531

0.731(2) 0.8086

0.826(2) 0.9393

0.164(7) 0.2189

1.402(1) 1.4062

0.705(2) 0.7186

0.671(2) 0.6505

1.388(1) 1.3510

0.04(4)i 0.0

0.11(1)i 0.0

1.139(1) 1.1401

1.195(1) 1.1953

0.178(9)i 0.0

2.6774(6) 2.7104

Ne

•

3.375 Å

3.832 Å

a Calculated values with the partial r0 geometry r = 3.400(2) Å and h = 83.8(3)° (see Fig. 1).

atom, rather than towards the C4 atom, as shown in Fig. 3. The three N–Ne, N–C4 and Ne–C4 distances there reported have been calculated from the rs coordinates of the three involved atoms. We also calculated the bond lengths of the heavy atoms in the aromatic ring (N–C2, C2–C3, Ne–C4) which resulted to be 1.338(4), 1.388(5) and 1.397(5) Å, respectively. They coincide, within the experimental errors, with the corresponding values of isolated pyridine [25]. A partial (van der Waals) r0 structure was obtained by fitting the distance r (distance of Ne from the center of mass of pyridine) and the angle h of Fig. 1 to the 21 available rotational constants, while keeping the geometry of Pyr fixed to that of the isolated molecule [25]. These values [r = 3.400(2) and h = 83.8(3)] are reported in a note at the bottom of Table 4.

N

•

•C2 2.814 Å C3 •

• C4

Fig. 3. The N–Ne, N–C4 and Ne–C4 rs distances prove unambiguously that the Ne atom is considerably shifted towards the N atom, rather than towards the C4 atom.

5. Energetics Information on the vibrational energies of the van der Waals motions and on the dissociation energy (ED) of the complex was reported, obtained from the changes of planar moments of iner-

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B. Velino, W. Caminati / Journal of Molecular Spectroscopy 251 (2008) 176–179 Table 5 Comparison of molecular parameters of the series Pyr  RG (RG = He, Ne, Ar and Kr) and pyridine Parameter

Pyr  Hea

Pyr  Neb

Pyr  Arc

Pyr  Krd

Pyre

A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz HJ/kHz HJK/kHz HKJ/kHz HK/kHz vaa(14N)/MHz vbb(14N)/MHz vcc(14N)/MHz Pa/u Å2 Pb/u Å2 Pc/u Å2

3875.209(5) 3753.251(5) 2978.437(9) 124.1(6) 120(4) 245(3) 4.3(3) 0.16(10) 3.05(4) 6.6(5) — 4.1(6) 4.789(8) 1.447(8) 3.342(5) 86.9585 82.7208 47.6925

3011.463(3) 1876.4048(7) 1858.0647(9) 20.47(3) 90.6(2) 104.9(2) 0.528(3) 29(2) 0.00156(6) 0.0206(5) 0.0624(7) 0.0383(4) 3.257(4) 4.706(5) 1.450(5) 85.2418 82.5790 186.7554

2990.3387(2) 1207.8635(1) 1199.3283(1) 3.443(3) 19.676(7) 22.105(2) 0.0382(11) 0.033(7) 0.000036(5) 0.00096(3) 0.00315(10) 0.00232(10) 3.3648(25) 4.8053(20) 1.4406(25) 85.9907 83.0132 335.3943

2986.7034(1) 806.9294(1) 803.0235(2) 1.37(5) 8.74(7) 9.4(2)

6039.2475(3) 5804.9075(3) 2959.2097(3) 1.3466(2) 2.2483(2) 1.0139(2) 0.0069(1) 0.0206(1)

3.3743(12) 4.8164(11) 1.4421(14) 86.1280 83.0817 543.2173

4.908(3) 1.434(3) 3.474(3) 87.0779 83.7017 0.0193

a b c d e

Ref. [5]. This work. Refs. [6] (quadrupole coupling constants) and [9] (rotational and centrifugal distortion constants). Ref. [6]. Ref. [25a] (quadrupole coupling constants) and [25b] (rotational and centrifugal distortion constants).

tia of the complex with respect to isolated pyridine and from centrifugal distortion constants, in Ref [8]. ED was estimated to be 1.1 kJ mol1. We performed distributed polarizability model (DPM) calculations using the computer program RGDMIN [26]. The geometry of Pyr was fixed to its reported structure [25] and we found ED = 2.5 kJ mol1, more than the double of the value estimated from the experiments. These calculations also supplied the values of r and the angle h of Fig. 1 at the energy minimum, [r = 3.173 and h = 89.6], values slightly different with respect to the r0 indications.

Acknowledgment We thank the University of Bologna for financial support. References [1] [2] [3] [4] [5] [6] [7]

6. Conclusions Tanjaroon and Jäger in their paper on Pyr  He [5], reported in their Table V a comparison of the spectroscopic constants of the four Pyr  RG (RG = He, Ne, Ar, Kr) complexes. Now we updated that Table (Table 5 here), introducing the new parameters for Pyr  Ne, and the centrifugal distortion constants for Pyr  Ar from Ref. [9] and Pyr from Ref. [25b], which escaped the attention of Tanjaroon and Jäger. The most noticeable trends are: (1) In all complexes, but not Pyr  He, there is a shift of the principal axes of inertia with respect to isolated pyridine. (2) The centrifugal distortion constants largely increase (up to two orders of magnitude for DJ) in going from RG = Kr to RG = He, due to the flatter potential energy surfaces and to the smaller reduced masses of the van der Waals motions. (3) the 14N vcc quadrupole coupling constant of Pyr  He, corresponding to vaa when RG = Ne, Ar and Kr, should have the same value of vcc of Pyr if the RG atom would lie rigidly along the Pyr c-axis; its decrease in going from Pyr to Pyr  Kr, Pyr  Ar and Pyr  Ne is outlining that the average position of the RG atom is more away from the c-axis as lighter the RG atom is. However, rather strangely, the vcc value of Pyr  He represents en exception to this trend. One can also note that the best sets of experimental parameters are obtained when both FTMW and mmw data are available, as in the case of Pyr  Ne and Pyr  Ar.

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