Infrared spectra of He–, Ne–, and Ar–C6D6

Chemical Physics Letters 610–611 (2014) 121–124

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Infrared spectra of He–, Ne–, and Ar–C6 D6 J. George a , A.R.W. McKellar b , N. Moazzen-Ahmadi a,∗ a b

Department of Physics and Astronomy, University of Calgary, 2500 University Drive North West, Calgary, Alberta T2N 1N4, Canada National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada

a r t i c l e

i n f o

Article history: Received 15 May 2014 In final form 27 June 2014 Available online 8 July 2014

a b s t r a c t Spectra of He–, Ne– and Ar–C6 D6 in the region of the 12 fundamental of C6 D6 are observed in a pulsed supersonic jet expansion using a tunable optical parametric oscillator laser source. These are the first reported infrared spectra of rare gas–benzene complexes and the first observation in any region for He– and Ne–C6 D6 . Two bands are studied for each complex: the 12 fundamental itself (≈2289 cm−1 ) and the 2 + 13 combination band (≈2276 cm−1 ) which are coupled by a Fermi resonance. Effective intermolecular separations of 3.593(1) and 3.455(1) A˚ are obtained for He– and Ne–C6 D6 , respectively, consistent with previous ultraviolet and microwave results for the analogous C6 H6 complexes. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Weakly-bound rare gas (Rg)–benzene clusters provide attractive model systems for progressive solvation of an organic molecule, and their spectroscopy with (at least partial) rotational resolution has an extensive history [1]. Thus, for example, electronic spectra have been obtained using laser induced fluorescence, coherent ion dip, or resonant enhanced two photon ionization techniques for the dimers containing benzene together with He [2,3], Ne [4], Ar [5–7], Kr [8], and Xe [8], nonlinear Raman spectroscopy has been applied to He [9] and Ar [10], and microwave spectroscopy to Ne [11,12], Ar [11,13], Kr [14], and Xe [11]. The Rg–benzene dimers are found to have a simple structure in which the rare gas atom is on the average located on the C6 symmetry axis at a distance of about r0 = 3.4–3.8 A˚ (depending on the species) from the plane of the benzene molecule. This reduces the point group symmetry from D6h for benzene monomer to C6v for Rg–benzene dimers. However, there have been no previous studies of Rg–benzene complexes in the infrared region. The present Letter reports such a study, based on observations of He–, Ne–, and Ar–C6 D6 in the region of the 12 C–D stretching band of C6 D6 near 2289 cm−1 . There have been very few previous spectroscopic observations of He–benzene, and our work also represents the first observation in any spectral region of the He– and Ne–C6 D6 isotopologues. There is a strong anharmonic (Fermi) resonance between 12 and 2 + 13 (≈2276 cm−1 ), so we actually observe two bands for

∗ Corresponding author. E-mail address: [email protected] (N. Moazzen-Ahmadi). http://dx.doi.org/10.1016/j.cplett.2014.06.059 0009-2614/© 2014 Elsevier B.V. All rights reserved.

each dimer. This 12 fundamental region of C6 D6 was analyzed in detail by Plíva et al. [15] in 1994. While carrying out the present work on Rg–C6 D6 complexes, we also observed the C6 D6 monomer spectrum with an effective rotational temperature of about 2.5 K, as reported recently [16]. Although our observed 12 band was reproduced very well by the Plíva et al. parameters, the 2 + 13 combination band turned out to be located 1.22 cm−1 higher than the band center determined by them [15]. In [16], we reported parameters from simple uncoupled symmetric rotor fits to these two bands, because the low rotational temperature precluded a comprehensive reanalysis.

2. Results Spectra were recorded using a previously described pulsed supersonic slit jet apparatus [16–18], which was probed with a Lockheed Martin Aculight Argos optical parametric oscillator source. The expansion gas mixtures contained approximately 0.1% C2 D6 . The carrier gas was He for He–C6 D6 , a 1:2 mixture of Ne:He for Ne–C6 D6 , and a 1:5 mixture of Ar:He for Ar–C6 D6 . Backing pressures were about 9 atm. Simultaneous etalon and N2 O reference gas spectra were recorded for frequency calibration. The pgopher computer program [19] was used for spectral simulation and fitting. As usual for symmetric top molecules, the rotational constant for motion around the symmetry axis was not easily determined. In the analyses below, we assumed that this parameter (A or C) for the ground vibrational state of Rg–C6 D6 complexes is equal to that of C6 D6 itself, 0.078496 cm−1 , as determined by Plíva et al. [15,20].

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2.1. He–C6 D6 Three previous publications on the He–benzene complex are especially relevant here. The first, from 1979, is a pioneering laser induced fluorescence study of the electronic spectrum by Beck et al. [2], who studied the S1 ← S0 61 0 vibronic band (≈38 610 cm−1 ) in a pulsed supersonic jet expansion with an effective rotational temperature of about 0.3 K. The spectrum was remarkably clear and clean, but the accuracy of their fitted parameters was limited by their spectral resolution (≈0.04 cm−1 ). Recently, the same spectrum was re-examined by Hayashi and Ohshima [3] under similar conditions, but with greatly improved resolution (<0.01 cm−1 ) using resonant enhanced two photon ionization spectroscopy. Their analysis yields a value of r0 = 3.60 A˚ for the effective distance between the He atom and the benzene plane. The third key paper, by Lee et al. [9], describes nonlinear Raman spectroscopy results, together with an analytical potential energy surface for He–C6 H6 which is based on high-level ab initio calculations. This potential ˚ and also gives us surface yields a very similar value for r0 (3.61 A), a value of re = 3.16 A˚ for the equilibrium intermolecular separation, that is, the global minimum of the surface itself, at an energy of −89.6 cm−1 . We observed He–C6 D6 spectra accompanying both the 12 and 2 + 13 bands, as illustrated in Figure 1. The stronger lines here can all be assigned to the C6 D6 monomer, as described previously [16]. But among the monomer lines are transitions due to He–C6 D6 as shown by the simulated spectra. The dimer bands are roughly 6 times weaker than the monomer bands. We assigned and fitted 90 lines in terms of 140 transitions, fixing C for the ground state at its monomer value (see above) and varying a total of 15 parameters. The fit utilized the symmetric rotor Hamiltonian built in to the pgopher program [19], which is equivalent to the standard form as given, for example, in [21]. This was a simultaneous fit, so that both observed bands helped to determine the ground state parameters. The resulting molecular parameters are given in Table 1, and the assignments themselves are given in the supplementary data section. Note that it was possible to determine a value for the l-type doubling parameter q for the 2 + 13 upper state (2275.96 cm−1 band), but not for the 12 fundamental (nor for any of the

2288.0

2275.0

2288.5

2275.5

2289.0

2276.0

Wavenumber / cm-1

2289.5

2276.5

2277.0

Figure 1. Observed (upper traces) and simulated (lower traces) spectra of He–C6 D6 in the 12 fundamental region. The simulated spectra are based on the parameters in Table 1 with a temperature of 2.5 K and a gaussian line width of 0.0029 cm−1 .

Ne– or Ar–C6 D6 bands below). The simulated spectra in Figure 1 are based on these parameters and assume a temperature of 2.5 K and a line width of 0.0029 cm−1 . They are calculated using the parameters from Table 1, and include the appropriate spin weights, which are: 65 for K = 0; and 116:124:119:124:116:130 for K = 1 + 6n:2 + 6n:3 + 6n:4 + 6n:5 + 6n:6 + 6n, where n = 0, 1, 2, 3, 4, . . .. 2.2. Ne–C6 D6 The Ne–C6 H6 dimer has been studied by electronic [4] and microwave [11,12] spectroscopy, but there are no previous reports of Ne–C6 D6 . Our Ne–C6 D6 spectra are shown in Figure 2. The assignments (given in the supplementary data section) were more

Table 1 Molecular parameters for C6 D6 and Rg–C6 D6 complexes.a C6 D6 [15,20]b 

B C (A )c DJK  DJ 

0.15701902 0.078496 −4.174e−8 2.50e−8

He–C6 D6

Ne–C6 D6

0.107559(23) 0.078496 1.47(69)e−6 0.1(51)e−8

0.056173(15) 0.078496 2.4e−6d 5.3e−7d

Ar–C6 D6 0.03710704 0.078496 4.847e−7 8.92e−8

0 B’ C’/A’ ’ DJK ’ DJ ’

2275.8776(1) 0.1567464(68) 0.0783580(50) −0.33501 −4.174e−8 2.50e−8

2275.9621(2)e 0.107577(22) 0.078350(18) −0.34140(39) −2.83(55)e−6 3.77(44)e−6

2275.9210(2) 0.056156(19) 0.078374(12) −0.34504(31) 2.4e−6 5.3e−7

2275.8498(1) 0.0370964(28) 0.0783220(78) −0.33816(11) 5.45(86)e−7 3.3(20)e−8

0 B’ C’/A’ ’ DJK ’ DJ ’

2288.7395(2) 0.1568001(48) 0.0784018(37) −0.17017 −4.174e−8 2.50e−8

2288.7326(2) 0.107632(31) 0.078401(26) −0.18180(33) −4.79(91)e−6 4.98(76)e−6

2288.6727(2) 0.056157(15) 0.078427(9) −0.18648(22) 2.4e−6 5.3e−7

2288.5337(1) 0.0371050(23) 0.0784608(76) −0.18347(10) 4.25(65)e−7 5.0(15)e−8

a Units of cm−1 , except for  which is dimensionless. Uncertainties in parentheses are 1 expressed in units of the last quoted digit; parameters without uncertainties were fixed. b DK = 1.88e−8 cm−1 for all C6 D6 states. c Parameter for rotation about the C6 symmetry axis, assumed in the ground state to be equal to that of C6 D6 . This is labeled C for C6 D6 and He–C6 D6 , or A for Ne– and Ar–C6 D6 . d Scaled from known [11] Ne–C6 H6 parameters by the ratio for Ar–C6 D6 and Ar–C6 H6 (see text). e Also: q = −9.2(20)e−5 for the 2275.9621 cm−1 state of He–C6 D6 .

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Table 2 Effective intermolecular distances for Rg–benzene dimers determined from experimental B-values (in Å).a

He Ne Ar

2288.0

2288.5

2289.0

C6 H6

C6 D6

3.602(3)b 3.462d 3.586d,e

3.594(1)c 3.454(1)c 3.579d,e

a Uncertainties in parentheses are 1 expressed in units of the last quoted digit, based on the B-value uncertainty. Values without uncertainties are from microwave spectra where B is very precise. b Ref. [3]. c Present work. d Ref. [11]. e Ref. [13].

2289.5

Table 3 Summary of vibrational shifts for the 12 and 2 + 13 bands for Rg–C6 D6 complexes, and separation of band origins (in cm−1 ).a

2275.0

2275.5

2276.0

-1

2276.5

2277.0

Wavenumber / cm

Figure 2. Observed and simulated spectra of Ne–C6 D6 in the 12 fundamental region.

C6 D6 He–C6 D6 Ne–C6 D6 Ar–C6 D6 a b

difficult here than for He–C6 H6 because of the presence of more extraneous (i.e. unassigned) lines, which are probably due to C6 D6 monomer hot bands. A total of 63 lines were fitted in terms of 199 transitions in a simultaneous analysis to give the Ne–C6 D6 parameters listed in Table 1. The ground state constant for rotation around the symmetric top axis (this is now labeled A) was again fixed at the monomer C-value. We noted that the microwave values of the distortion constants DJK and DJ are quite similar for Ar–C6 H6 and Ar–C6 D6 [13], and therefore estimated values of these constants for Ne–C6 D6 by simple scaling of those of Ne–C6 H6 [11] according to their ratios for the Ar complexes. These scaled values were held fixed in the analysis for the ground and excited states. With these constraints, there were 9 adjustable parameters in the fit (see Table 1).

Shift 12 band

Shift 2 + 13 band

12 minus (2 + 13 )b

0.0 −0.0069 −0.0668 −0.2058

0.0 +0.0845 +0.0434 −0.0278

12.862 12.771 12.752 12.684

Uncertainties in parentheses are 1 expressed in units of the last quoted digit. This is the difference between the 12 and the 2 + 13 band origins.

2.3. Ar–C6 D6 Ar–benzene dimers have been extensively studied in the past; see Fernández and co-workers [22] for a recent high level ab initio calculation and references to other work. In particular, the microwave spectrum of Ar–C6 D6 was reported by Brupbacher and Bauder [13]. Thus accurate ground state constants (B, DJK , DJ ) were already available for this species, making our assignment of the spectra shown in Figure 3 relatively easy. Since our expansion gas was 80% helium, some He–C6 D6 transitions are also weakly visible in Figure 3, most notably around 2275.97 and 2288.65–2288.80 cm−1 . A total of 89 lines involving 307 transitions were fitted to give the Ar–C6 D6 parameters listed in Table 1. With the ground state parameters all fixed, a total of 8 excited state parameters were adjusted, including DJK and DJ for both upper states. 3. Discussion and conclusions

2288.0

2275.0

2275.5

2288.5

2276.0

Wavenumber / cm-1

2289.0

2276.5

2289.5

2277.0

Figure 3. Observed and simulated spectra of Ar–C6 D6 in the 12 fundamental region.

Looking at the results in Table 1, we note that the Coriolis interaction parameter, , tends to increase slightly in magnitude when a rare gas atom is added to C6 D6 . As might be expected, the increase is larger for Ne than for He, but then it tends to fall back a bit for Ar. This is quite similar to the trend observed for  in the upper state of the S1 ← S0 61 0 vibronic bands of Rg–C6 H6 complexes [4,5]. We also note that the changes in rotational constants upon vibrational excitation to the 12 and 2 + 13 states for the Rg complexes are small, and tend to be similar to those of C6 D6 itself. A key result from an Rg–benzene spectrum is the B-value, which directly gives an intermolecular separation (with the usual assumption that the monomer structure remains unchanged in the dimer). Experimental results for the center of mass separations between He, Ne, or Ar and C6 H6 or C6 D6 are summarized in Table 2. To determine the present C6 D6 results, we assumed the benzene structure in the complexes remained unchanged at that given by Doi et al. ˚ As expected, [23], with R(C–C) = 1.39711 A˚ and R(C–H) = 1.08066 A. the separations for the C6 D6 complexes are all slightly smaller than

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for C6 H6 , as expected due to the larger reduced mass of the deuterated complex and the effects of anharmonicity. And the differences increase in going from Ar to Ne to He as these effects become larger. Table 2 thus confirms that our new B-values for He– and Ne–C6 D6 are entirely consistent with previous results [3,11] for He– and Ne–C6 H6 . Vibrational shifts of the Rg–C6 D6 band origins relative to the C6 D6 monomer are summarized in Table 3. These shifts are fairly small, with the largest in magnitude being −0.2 cm−1 for the 12 band of Ar–C6 D6 . In terms of direction, the shifts tend to be more positive (blue-shifted) for He and increasingly more negative (redshifted) going to Ne and to Ar. This mirrors a common trend for van der Waals complexes, which can be rationalized in terms of attractive forces becoming more dominant as the rare gas atom becomes progressively heavier and more polarizable. The last column of Table 3 shows that there is a small but regular decrease in the separation between the 12 and 2 + 13 band origins in going from the C6 D6 to the He–, Ne–, and then Ar–C6 D6 dimer. This could be interpreted as indicating that, as the rare gas mass increases, there is a steady decrease in the magnitude of the Fermi interaction coupling these vibrations. On the other hand, it could be interpreted as simply being due to differing vibrational shift effects for the two vibrational modes. Acknowledgments The authors gratefully acknowledge the financial support of the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada. We thank M. Rezaei and M. Yousefi for assistance with the measurements.

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