ARTICLE pubs.acs.org/JPCA
Spectroscopic and Theoretical Study of the Weakly Bound H2HCCCN Dimer Julie M. Michaud, Wendy C. Topic, and Wolfgang J€ager* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
bS Supporting Information ABSTRACT: Rotational spectra of the H2HCCCN complex were studied using a pulsed-nozzle Fourier transform microwave spectrometer. Complexes containing the main and several minor isotopologues of cyanoacetylene (HCCC15N, DCCCN, and various 13C containing isotopologues) and the two spin isomers of the H2 molecule (paraH2 and orthoH2) were investigated. Transitions of complexes with 14N and D containing isotopologues have nuclear quadrupole hyperfine structures, which were measured and analyzed. Transitions of orthoH2 molecule containing complexes show additional hyperfine structures due to nuclear magnetic proton spinproton spin coupling of the hydrogen nuclei in the H2 molecule. For orthoH2HCCCN, both strong a- and weaker b-type transitions were measured and analyzed using a semirigid asymmetric rotor model. For the paraH2HCCCN complex, only a-type transitions could be observed. The dimer complexes are floppy and have near T-shaped structures. Intermolecular interaction potential energy surfaces were calculated for H2HCCCN using the coupled-cluster method with single and double excitations and noniterative inclusion of triple excitations [CCSD(T)]. Three orientations of the hydrogen molecule within the complex were considered. Equal weighting of the surfaces corresponding to the three hydrogen orientations provided an averaged potential energy surface. Bound-state rotational energy levels supported by the surfaces were determined for the different hydrogen orientations, as well as for the averaged surface. Simple scaling of the surfaces improved the agreement with the experimental results and produced surfaces with near spectroscopic accuracy.
1. INTRODUCTION A number of very weakly bound complexes and clusters containing helium atoms or hydrogen molecules have been investigated with high-resolution spectroscopic and ab initio methods (for example, refs 131). These systems are characterized by shallow and broad interaction potential wells, by relatively high lying zero-point energy levels, and by the resulting large amplitude intermolecular motions. In studies of helium atoms containing medium sized clusters of the type HeN molecule, it was observed that the rotational constant B of the cluster increases with increasing number of helium atoms at some critical cluster size N. This nonclassical behavior indicates a decoupling of the helium density from the rotational motion of the dopant molecule and is now generally accepted to be a microscopic manifestation of superfluidiy. The critical cluster size N depends sensitively on the particular heliummolecule interaction potential; for example, the turnaround in B occurs at N = 9 for HeNOCS,3 N = 10 for HeNHCCCN,11 and N = 3 for HeNCO.1,26,31 Helium is thus far the only substance for which superfluidity has been observed under laboratory conditions. It appears that the only other promising candidate for a superfluid is molecular hydrogen, H2. Hydrogen occurs as two different spin isomers, i.e., paraH2 with total nuclear spin quantum number Itotal = 0 and r 2011 American Chemical Society
orthoH2 with Itotal = 1. The superfluid transition temperature for paraH2 has been predicted to be 6 K.32 However, paraH2 freezes at 13.8 K and attempts to supercool paraH2 have been unsuccessful.3236 Toennies and co-workers have studied the infrared spectrum of (paraH2)NOCS embedded in superfluid helium nanodroplets.3741 The absence of certain spectroscopic features was interpreted as indication of superfluidity in paraH2. The observation of a turnaround in the B rotational constant in small (paraH2)Nmolecule clusters, similar to the findings in HeN molecule clusters, would provide, in combination with theoretical simulations, an unambiguous confirmation of a superfluid phase of paraH2. Quantum Monte Carlo simulations of (paraH2)NOCS clusters predict a turnaround in B at N = 16.42 The spectroscopic characterization of (H2)1molecule dimers is a prerequisite step for the investigation of larger clusters. Also, theoretical simulations of the larger clusters require accurate potential energy surfaces of the H2molecule interaction. Special Issue: David W. Pratt Festschrift Received: December 13, 2010 Revised: March 6, 2011 Published: March 29, 2011 9456
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H2molecule systems studied so far include H2OCS (infrared and microwave spectra)5,15,43,44 and H2CO45 and H2N2O6 (infrared spectra). Accurate potential energy surfaces exist for H2OCS,46 H2CO,47 H2N2O,48 and H2CO2.49 Here, we report a microwave rotational spectroscopic and theoretical study of the H2HCCCN dimer. Complexes containing orthoH2 and paraH2, as well as several minor isotopologues of HCCCN (DCCCN, HCCC15N, and various 13Cisotopologues) were investigated. The spectra were analyzed to yield rotational, centrifugal distortion, and hyperfine coupling constants, which, in turn, were interpreted in terms of molecular structure and dynamics. An intermolecular ab initio potential energy surface was constructed at the CCSD(T) level of theory. We determined the bound state energy levels supported by this surface and calculated the corresponding transition frequencies. The potential energy surface was scaled to achieve a good agreement with the experimental transition frequencies.
2. EXPERIMENTAL DETAILS Rotational spectra of H2HCCCN were collected using a Balle-Flygare type pulsed-jet Fourier transform microwave spectrometer50 and a microwavemicrowave double resonance spectrometer.51 The details of these instruments have been described previously.52,51 The synthesis of HCCCN was done following the method of Moreau and Bongrand,53 as modified by Miller and Lemmon.54 Deuterated cyanoacetylene (DCCCN) was prepared in a manner similar to that reported by Mallinson and Fayt,55 as described previously.12 Isotopically enriched NH3 (10% 15NH3, Cambridge Isotope Laboratories, Inc.) was used to synthesize HCCC15N. The gas samples were composed of low concentrations of HCCCN (0.030.1%) and 13% of hydrogen gas or enriched para-hydrogen gas. The backing gas used for the studies of orthohydrogen containing dimers was helium or neon. A neon expansion was used for the study of transitions of orthoH2 HCCCN originating from rotationally excited energy levels. Generation of the para-hydrogen containing dimers required helium as a carrier gas. This is consistent with the order of binding energies obtained from the H2OCS dimer studies by Yu et al.15,44 The enriched para-hydrogen was obtained using a home-built converter with a chromium oxide catalyst, as described previously.43 Initially, gas samples containing enriched paraH2 were prepared by first filling cyanoacetylene into the sample cylinder, followed by enriched paraH2 and finally helium backing gas. In this procedure, the enriched paraH2 interacted with the chromophore gas for several minutes (up to >10 min) prior to being diluted by helium. We noticed that the signal enhancement seen for the paraH2HCCCN transitions was not as strong as expected from the OCS studies.43,44 We assume that backconversion from paraH2 to orthoH2 occurred in the H2/HCCCN mixture, catalyzed by the inhomogeneous magnetic field produced by the nonzero nuclear spin of the 14N nucleus with nuclear spin quantum number I(14N) = 1.56 Previous studies have shown that a collision of paraH2 with a paramagnetic molecule such as O2 can lead to efficient backconversion to orthoH2.56 The interaction of a magnetic field gradient with a spin isomer has been shown to induce spin conversion, and collisions can have a major effect on the conversion rate.5759 We note, however, that the magnetic moments of both the proton and the
Figure 1. (a) A composite experimental spectrum of the JKaKc = 101000 transition of orthoH2HCCCN, obtained with 100 averaging cycles. Each 14N hyperfine component was measured in a separate experiment. (b) A composite experimental spectrum of the JKaKc = 111000 transition of orthoH2HCCCN, obtained with 250 averaging cycles, using neon as backing gas and a microwave power amplifier. For both spectra, the time domain signals were recorded at 20 ns sampling interval to obtain 8k data points. The data set was supplemented with 8k zeros before Fourier transformation. Line frequencies were obtained from an interpolation procedure.
nitrogen nucleus are significantly smaller than that of O2. In the current studies, substantial signal enhancement was achieved when a sample of a trace amount of HCCCN in high-pressure helium was added to a gas cylinder containing the enriched sample of paraH2 gas.6,8 An important aspect to this sample preparation method is that the chromophore and the enriched paraH2 gas are mixed in the presence of excess helium. The new sample preparation led to signal strengths comparable to those seen with paraH2OCS dimers.
3. EXPERIMENTAL RESULTS AND ANALYSES An initial transition frequency prediction for H2HCCCN was obtained by calculating the fractional changes for the JKaKc = 101000 transitions of the HeOCS (ref 16) and H2OCS (ref 44) complexes and assuming this change to be approximately the same for the corresponding HCCCN containing complexes. Within 20 MHz of the predicted frequency, a transition was located which has 14N nuclear quadrupole hyperfine structure and additional hyperfine structure which is consistent with proton spinproton spin coupling due to an orthoH2 molecule. This transition was assigned to be the JKaKc = 101000 transition of orthoH2HCCCN; see Figure 1a. The b-type transitions for orthoH2HCCCN and orthoH2HCCC15N were detected using double resonance decoherence experiments.51 The JKaKc 9457
dx.doi.org/10.1021/jp111812k |J. Phys. Chem. A 2011, 115, 9456–9466
The Journal of Physical Chemistry A
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Table 1. Determined Spectroscopic Parameters for the orthoH2HCCCN, orthoH2HCCC15N, and orthoH2 DCCCN Dimers (all values are in MHz, unless stated otherwise) orthoH2HCCCN orthoH2DCCCN orthoH2HCCC15N A
20434.84172(53)a
B
4531.23799(81)
C
3585.44274(53)
(B þ C)/2
Figure 2. A composite experimental spectrum of the JKaKc = 101000 transition of paraH2HCCCN, obtained with 100 averaging cycles. An enriched paraH2 sample was used to obtain this spectrum. The time domain signals were recorded at 20 ns sampling interval to obtain 8k data points. The data set was supplemented with 8k zeros before Fourier transformation.
= 101000 a-type transition was monitored while the frequency of the pump microwave radiation was swept through the region where the JKaKc = 111000 b-type transition was expected. The measurement of the b-type transitions required an amplifier for the microwave excitation pulses in addition to longer pulse widths. This is consistent with a larger a- than b-dipole moment component. The JKaKc = 111000 b-type transition of orthoH2HCCCN is shown in Figure 1b. The weaker signal and the difference in the 14N nuclear quadrupole hyperfine structure when compared to Figure 1a substantiate the assignment. A sample consisting of neon (35 atm) with 1.5% H2 gas and
