ARTICLE pubs.acs.org/JPCA
CH 3 3 3 π Interactions in the CHBrF2 3 3 3 HCCH Weakly Bound Dimer Daniel A. Obenchain, Brandon J. Bills, Cori L. Christenholz, Lena F. Elmuti, Rebecca A. Peebles, and Sean A. Peebles* Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, Illinois 61920, United States
Justin L. Neill and Amanda L. Steber Department of Chemistry, University of Virginia, McCormick Road, PO Box 400319, Charlottesville, Virginia 22904, United States
bS Supporting Information ABSTRACT: The microwave spectra of four isotopologues of the CHBrF2 3 3 3 HCCH weakly bound dimer have been measured in the 618 GHz region using chirped-pulse and Balle Flygare Fourier-transform microwave spectroscopy. Spectra of 13CH79BrF2 and 13CH81BrF2 monomers have also been measured, and spectroscopic constants are reported. Measurement of spectra for the 79Br and 81Br isotopologues of CHBrF2 complexed with both 12C2H2 and 13C2H2 have allowed the determination of a structure with Cs symmetry for this complex. CHBrF2 interacts with the triple bond of acetylene via a CH 3 3 3 π contact (R(H 3 3 3 π) = 2.670(8) Å) with the Br atom lying in the ab plane, located 3.293(40) Å from a hydrogen atom of the HCCH molecule. The structure of CHBrF2 3 3 3 HCCH has been compared with recently studied related acetylene complexes, including a comparison with (and further structural analysis of) the CHClF2 3 3 3 HCCH complex.
I. INTRODUCTION Recent studies in our lab have focused on CHClF2,1 CH2ClF,2 and CH2F23 complexed with acetylene, and we report here on a bromine analogue of the former, namely CHBrF2 3 3 3 HCCH. This forms a logical extension of our studies of simple molecules complexed with halogenated methanes that show potential for the formation of CH 3 3 3 X interactions (where X is an electronegative element such as oxygen or fluorine or a π system). The goal is to gather data to allow a better understanding of the effect of changes in the degree and identity of halogenation upon structural parameters such as the CH 3 3 3 π contact distances between the two subunits. The previously studied complexes of acetylene with CHClF2,1 CH2ClF,2 and CH2F23 all exhibit a CH 3 3 3 π interaction, with distances of 2.724(7) (see Discussion), 3.236(6), and 3.363(14) Å, respectively. The latter two complexes have a double CH 3 3 3 π interaction, with both H atoms of the substituted methane straddling the symmetry plane of the complex, and hence have longer H 3 3 3 π contact distances than the CHClF2 3 3 3 HCCH complex in which the lone chlorodifluoromethane H atom lies in the ab symmetry plane. It is expected that a similar situation should be observed in the present case (with the H and Br atoms lying in the symmetry plane). A recent ab initio investigation4 of the structure and interaction energies of acetylene complexed with CHClF2 and CHF3 revealed that a somewhat different CHClF2 3 3 3 HCCH structure could be obtained that is consistent with our previously reported rotational constants for CH35ClF2 3 3 3 H12C12CH and incorporates an MP2/aug-cc-pCVQZ CHClF2 monomer structure. Some key intermolecular parameters differed from our original r 2011 American Chemical Society
structural fit, highlighting the sensitivity of data to the monomer structures and emphasizing the importance of experimental rotational constants and accurate structural data as an important benchmark for the current generation of computational, particularly density functional, approaches. In the remainder of this article, the present results for CHBrF2 3 3 3 HCCH will be discussed and compared to our previous work and we will also revisit the structure of the CHClF2 3 3 3 HCCH complex in light of the new computational data.4
II. EXPERIMENTAL METHODS Ab initio optimizations were carried out on likely geometries for the CHBrF2 3 3 3 HCCH complex at the MP2/6-311++G(2d,2p) level using Gaussian 03.5 MP2 densities were used to calculate molecular properties, and the OUTPUT = PICKETT keyword provided nuclear quadrupole coupling tensors and dipole moment components in the principal axis frame. Harmonic frequency calculations allowed characterization of all stationary points and provided zero-point energy corrected relative stabilities. The 718 GHz microwave spectrum of CHBrF2 3 3 3 HCCH was initially scanned using the chirped-pulse Fourier-transform microwave (FTMW) spectrometer at the University of Virginia (UVA).6 A sample of 0.75% CHBr2F (>99%, Synquest Laboratories) and 1% HCCH (Praxair, 99.6%) diluted in He was pulsed through three parallel 1 mm orifice nozzles into the vacuum Received: September 6, 2011 Revised: October 6, 2011 Published: October 06, 2011 12228
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Table 1. Predicted Spectroscopic Constants and Relative Energies of Four ab Initio (MP2/6-311++G(2d,2p)) Structures of CH79BrF2 3 3 3 H12C12CHa
Figure 1. Ab initio structures IIV for CHBrF2 3 3 3 HCCH, optimized at the MP2/6-311++G(2d,2p) level. The inset for structure II shows this structure viewed after rotation by 90° about a vertical axis in the plane of the page. Structure I corresponds to the species observed experimentally; see Figure 3 for the definition of the fitted structural parameters.
chamber of the spectrometer with a backing pressure of 2 atm. Ten free induction decays (FIDs), with 10 μs duration, were collected per gas pulse. The microwave excitation pulse train, a series of 10 sequential chirped pulses with 1 μs duration and 25 μs separation, was amplified by a 200 W traveling wave tube amplifier (AR 200T8G18A). When using the pulse train to record 10 FIDs for each sample injection cycle, it is not possible to use the 1 kW amplifier employed in previous measurements6 without damaging the diode that protects the receiver. The repetition rate of the pulsed nozzles was 0.8 Hz, which is currently limited by the data processing capability of the digital oscilloscope. A total of 200 000 FIDs were averaged together in the time domain and Fourier transformed to provide the final frequency-domain spectrum. The 79Br and 81Br isotopic species were identified in roughly equal intensities in the broadband scan, and transitions were fitted with Pickett’s SPFIT program7 using Kisiel’s AABS package8 to facilitate the initial rotational spectrum assignments. Rotational transitions lying outside the frequency range of the broadband instrument (as well as spectra for all additional isotopic species) were measured using the BalleFlygare resonant cavity FTMW spectrometer9 at Eastern Illinois University (EIU).10,11 Initial identification of spectra for the CHBrF2 3 3 3 H13C13CH species was made using an isotopically enriched acetylene (H13C13CH) sample (99% 13C, Isotec) on the 480 MHz bandwidth chirped-pulse Fourier-transform microwave spectrometer12 at EIU in order to facilitate the assignment and conserve sample; all measurements of transition frequencies used in the fitting process were then made on the BalleFlygare FTMW spectrometer. Samples used at EIU were prepared using about 1% of each component in first-run He/Ne (17.5%:82.5% He/Ne, BOC gases) at a total backing pressure of 22.5 atm.
III. RESULTS A. Ab Initio Calculations. Ab initio optimizations identified four possible structures (IIV, Figure 1) and predicted rotational
structure
structure
structure
structure
I
II
III
IV
experiment
A (MHz)
2257
2451
4637
7444
2273.425(4)
B (MHz)
1262
1254
827
872
1243.8050(3)
C (MHz)
946
966
802
801
934.74155(28)
χaa(MHz) χbb (MHz)
130.9 397.9
63.9 284.1
489.9 220.9
310.2 252.5
172.792(5) 448.102(15)
χcc (MHz)
267.0
220.2
268.9
57.8
275.310(10)
χab (MHz)
299.8
353.2
202.1
110.4
230.81(17)
χac (MHz)
0
57.3
0
353.0
0
χbc (MHz)
0
98.7
0
70.1
0
μa (D)
1.71
1.30
0.79
0.23
μb (D)
0.43
0.67
1.34
1.35
μc (D) ΔE (cm1)b
0.00 0
0.79 25
0.00 87
0.47 329
See text for computational details. b ΔE gives the relative energies (zero-point energy corrected).
a
Table 2. Spectroscopic Constants for 13C Isotopologues of the CHBrF2 Monomera 13
CH79BrF2
13
CH81BrF2
A (MHz)
10167.9187(15)
10167.7455(17)
B (MHz)
2885.3333(14)
2858.3741(13)
C (MHz)
2350.00625(25)
2332.09160(26)
3/2χaa (MHz) 1/4(χbb χcc) (MHz)
783.359(12) 9.4118(29)
654.487(12) 7.8457(26)
χac (MHz)
186.1(4)
154.5(6)
Nb
20
21
Δνrms (kHz )c
7.6
7.0
Quartic and sextic centrifugal distortion constants were fixed at the values for 12CH79BrF2 and 12CH81BrF2 from ref 14. Transition frequencies are available as Supporting Information. b Number of hyperfine components in the fit. c Root-mean-square deviation of the fitted frequencies. a
constants, nuclear quadrupole coupling constants, and relative energies for these structures given in Table 1. Three of the four structures (IIII) exhibit some sort of CH 3 3 3 π interaction with the least stable structure (IV) possessing a T-shaped type geometry with a Br 3 3 3 π contact. All except structure III were identified as minima, with III possessing a single imaginary vibrational frequency. Structure I is determined to be the most stable at the zero-point energy corrected level, with II lying about 25 cm1 higher in energy and III 87 cm1 above I. Structure IV is considerably less stable, being 329 cm1 above the global minimum (I). The spectral assignment is consistent only with structure I, as will be discussed in detail later. B. Spectra. The broadband spectrum was dense and dominated by monomer transitions.13,14 The strongest signals in the broadband scan belonged to the CHBrF2 monomer, while the strongest dimer transitions were about 1% of this intensity. Previously unassigned 13CH79BrF2 and 13CH81BrF2 monomer transitions were observed in comparable intensity to the dimer transitions within the broadband scan and were fitted to provide 12229
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rotational constants for these species (Table 2) to allow confident elimination of 13CHBrF2 transitions from the scans; these rotational transition frequencies (as well as transition frequencies for CHBrF2 3 3 3 HCCH) are included as Supporting Information. Because all observed 13C monomer transitions were of low intensity, the data sets are quite small, so only rotational and nuclear quadrupole coupling constants were fitted, with centrifugal distortion constants being held fixed at values previously determined for the parent monomer species.14 This fit is sufficiently good to positively identify these transitions as belonging to the 13C species; however, attempts to use the 13C data to improve upon the CHBrF2 monomer structure reported in ref 13 were met with limited success, still giving large uncertainties in the parameters defining the position of the hydrogen atom.
Predictions of the dimer spectrum made using ab initio optimized structures (section III.A; Table 1) enabled relatively easy identification of the hyperfine split transitions (despite the magnitudes of predicted nuclear quadrupole coupling constants all being quite significantly underestimated at this level of calculation, by 24% and 11% for χaa and χbb, with the out-ofplane component (χcc) predicted most accurately (3%)). Although the percent differences for these constants are larger in the present case than for analogous Cl-containing species we have studied recently (where errors of less than 8% are more typical),1,2,15,16 they are still sufficiently accurate to provide the unambiguous assignment of hyperfine patterns in the observed spectrum. Assigned hyperfine transitions from the broadband scan were well resolved (Figure 2) and had signal-to-noise (S/N) ratios of ∼100, compared to a S/N approaching 10 000 for the strongest monomer signal. These dimer transitions were somewhat weaker on the UVA broadband instrument than for the analogous chlorine complexes (CH2ClF and CHClF2 with acetylene), which were scanned under similar conditions. Similar intensity observations were also noted on the resonant cavity instrument at EIU. A total of 132 and 129 nuclear quadrupole hyperfine components were assigned for the CH79BrF2 and CH81BrF2 species complexed with 12C2H2, respectively, spanning at least 35 separate a-type transitions per isotopologue (with J00 ranging from 3 to 8). These transitions were fit to a Watson A-reduction Hamiltonian in the Ir representation (Table 3).17 Inclusion of the χab off-diagonal term was necessary to achieve the desired quality of fit (for instance, the Δνrms for the CH79BrF2 3 3 3 12C2H2 species increases from 0.0034 to 0.36 MHz when χab is set to zero). Only a-type transitions were observed in the broadband scan, consistent with the relatively small (0.4 D) μb dipole moment component predicted for structure I, although all three rotational constants were well determined. For CHBrF2 3 3 3 H13C13CH, the data set was smaller due to the limited amount of enriched
Figure 2. Section of the 87908900 MHz region from the broadband scan showing various hyperfine components of the J = 3 r 4, Ka = 3 transitions for CH79BrF2 3 3 3 HCCH and CH81BrF2 3 3 3 HCCH; portions of the 13CH79BrF2 monomer 818 r 726 transition (marked with *) and the CH81BrF2 monomer 221 r 313 transition (marked with a +) also fall within this region.
Table 3. Spectroscopic Constants for the Four Isotopologues of CHBrF2 3 3 3 HCCH CH79BrF2 3 3 3 12C2H2
CH81BrF2 3 3 3 12C2H2
CH79BrF2 3 3 3 13C2H2
CH81BrF2 3 3 3 13C2H2
A (MHz) B (MHz)
2273.425(4) 1243.8050(3)
2259.183(5) 1239.8764(3)
2266.739(11) 1179.2732(9)
2252.507(11) 1175.4195(9)
C (MHz)
934.74155(28)
930.10190(26)
896.7676(5)
892.2953(5)
ΔJ (kHz)
1.5993(29)
1.5889(27)
1.451(11)
1.442(10)
ΔJK (kHz)
15.769(15)
15.316(15)
14.65(16)
14.55(14)
ΔK (kHz)
17.4(9)
19.7(7)
17.4 a
19.7 a
δJ (kHz)
0.4589(17)
0.4556(14)
0.394(7)
0.403(7)
δK (kHz)
2.41(8)
2.67(7)
2.41 a
2.67 a
3/2χaa (MHz) 1/4(χbb χcc) (MHz)
259.188(8) 180.853(4)
211.541(8) 150.256(4)
264.795(27) 181.769(13)
216.700(27) 151.091(13)
χab (MHz)
272.34(13)
230.81(17)
267.76(25)
227.75(33)
χaa (MHz)
172.792(5)
141.028(6)
176.530(18)
144.467(18)
χbb (MHz)
448.102(15)
371.025(15)
451.803(53)
374.416(53)
χcc (MHz)
275.310(15)
229.998(15)
275.273(53)
229.948(53)
Nb
132
129
38
37
Δνrms (kHz)c
3.4
3.9
3.1
2.8
Pcc (u Å2) θza (deg)d
43.9769(4) 69.37(1)
43.9728(4) 69.98(1)
43.9746(9) 69.31(1)
43.9692(9) 69.36(2)
a Fixed at the value from the corresponding parent isotopologue. b Number of hyperfine components in the fit. c Root-mean-square deviation of the fitted frequencies. d θza is the angle between the z axis of the nuclear quadrupole coupling tensor and the a axis of the complex; see text for discussion.
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Table 4. Structures of the CHBrF2 and HCCH Monomers, Taken from refs 13 and 20, Respectively CHBrF2a
R||| 3 3 3 C (Å) ϕC||| 3 3 3 C (deg) θBrC 3 3 3 ||| (deg)
RCH (Å)
1.098(20)
RCBr (Å)
1.908(20)
RCF (Å)
1.347(20)
θHCBr (deg)
110.8(40)
θFCBr (deg)
110.6(10)
θFCF (deg) τFCBrX (deg)b
107.4(20) (59.39(50)
1.061(2) 1.203(2)
a
Uncertainties estimated by the authors of the current paper based on the variation between ref 13, an attempt at a new inertial fit of the monomer using additional 13C data from the current work, and a high level ab initio calculation from ref 14. b The dihedral angle is measured to a dummy atom that lies in the HCBr plane, forming a 90° CBrX angle and a 180° HCBrX dihedral angle. 13
C2H2 sample available, but the rotational constants were equally well determined. Although the linewidths of the transitions measured on the CP-FTMW instrument are considerably larger than those on the BalleFlygare instrument (fwhm ∼250 kHz (CP-FTMW) compared to ∼35 kHz (BalleFlygare FTMW)), the rootmean-square deviations of the fits for all isotopologues were less than 4 kHz. Fitted constants are summarized in Table 3, and full lists of assigned transition frequencies are available as Supporting Information.18 C. Structure. The consistency of values for the Pcc planar moments (eq 1) of all observed isotopic species (Table 3) provides a preliminary confirmation of the overall geometry as being similar to structure I. Pcc ¼ 1=2ðIa þ Ib Ic Þ ¼
∑i mi c2i
σb
Ia, Ib, and Icc
3.688(67)
76(14)
91.8(40)
0.598
Ia and Ibc
3.6834(67)
73.9(14)
91.71(40)
0.0588
Ia and Icc
3.696(15)
72.9(28)
91.48(86)
0.0842
Ib and Icc
3.688(15)
81.4(35)
91.98(89)
0.105
structure Id
3.622
78.9
92.5
See Figure 3 for additional derived structural parameters. The fit of Ia and Ib is the favored structure. b Standard deviation of the fit in u Å2. c Denotes the moments of inertia which were included in the structural fit. See text for further discussion of the fitting process. Quoted uncertainties are one standard deviation; however, more realistic uncertainties for the preferred fit of Ia and Ib, based on observed variations with variation in monomer structure, are R||| 3 3 3 C = ( 0.01 Å, ϕC||| 3 3 3 C = ( 4°, and θBrC 3 3 3 ||| = ( 1°. d Ab initio optimized structure (MP2/6311++G(2d,2p) level). Additional parameters of interest for all predicted structures may be found in Figure 1. a
HCCH RCH (Å) RCC (Å)
Table 5. Structures for the CHBrF2 3 3 3 HCCH Dimer Obtained by Fits of Various Combinations of Experimental Moments of Inertia to Intermolecular Structural Parameters.a
ð1Þ
Values of Pcc for all isotopic species (∼44.0 u Å2; Table 3) lie in the vicinity of what would be expected if only fluorine atoms lie out of the ab plane (in which case a value similar to Pbb of the CHBrF2 monomer (Pbb ≈ 44.807 u Å2)13 would be obtained). Of course, both structures I and III would be expected to have similar values of Pcc since they both have fluorine atoms straddling the symmetry plane, but structures II and IV can effectively be eliminated from consideration at this point in light of the Pcc values. Although these Pcc values alone do not distinguish between geometries I and III, an inspection of the predicted rotational constants for the two possible structures clearly favors structure I as the experimentally observed species; the A rotational constant is underestimated by only 0.7%, while B and C are overestimated by 1.5% and 1.2%, respectively. Further support for an ab plane of symmetry can be gained from a comparison of the out-of-plane component of the nuclear quadrupole coupling tensor. The dimer values of χcc (Table 3) between the two isotopologues are very close to (but about 4 MHz smaller in magnitude than) the relevant χbb components for the CHBrF2 monomer (279.80(18) MHz for CH79BrF2 and 233.85(19) MHz for CH81BrF2).13 Also, the ratio
[χcc(79Br)/χcc(81Br)] in the complex for both acetylene isotopologues (1.19701(10) for 12C2H2 and 1.19711(30) for 13C2H2) is in perfect agreement with the corresponding ratio [χbb(79Br)/ χbb(81Br)] for the monomer (1.1965(8))13 and of the ratio of nuclear quadrupole moments [Q(79Br)/Q(81Br)] = 1.1946(255).19 The structure of CHBrF2 3 3 3 HCCH is derived by a leastsquares fit of selected structural parameters to moments of inertia of the four isotopologues. If the CHBrF2 and HCCH monomer structures are assumed to be unchanged upon complexation (Table 4)13,20 and Cs symmetry is assumed, then only three parameters remain to be determined (Figure 3): the distance between the two monomer subunits (R(||| 3 3 3 C)) and the angles of the two monomers with respect to an intermolecular axis (ϕ(C||| 3 3 3 C) and θ(BrC 3 3 3 |||)). The structure for CHBrF2 given in ref 13 was used, despite some obvious flaws such as a long (1.098 Å) CH bond. Additional structure fits using either the high level ab initio structure of CHBrF2 presented in ref 14 or our refitted structure (section III.B.) led to some variations in the intermolecular structural parameters; however, each of the monomer structures has clear advantages and disadvantages over the others, so the choice of which monomer structure to use would be quite arbitrary. Thus, the published experimental CHBrF2 monomer structure has been used for the current work,13 with reasonable uncertainties based on using the different monomer structures incorporated into the final fitted dimer structure (see below and Table 5). Fitting was carried out using the University of Michigan implementation of Schwendeman’s STRFITQ program.21 Fits of all 12 moments of inertia led to high uncertainties in fitted parameters, particularly in the tilt angle of the acetylene subunit. The best fits (as judged by the smallest standard deviations, Table 5) were those using the (Ia and Ib) and (Ia and Ic) combinations of moments; the C 3 3 3 C distances in these two fits are very similar, and the CHBrF2 and HCCH tilt angles (θ and ϕ, respectively) agree to within the stated uncertainties, although errors in ϕ are significantly higher than errors in θ in both fits. Measurement of the two singly 13Csubstituted HCCH isotopologues would probably have improved the quality of determination of this parameter, but only 13 C2H2 was available. Structural parameters derived from the fit using Ia and Ib moments will be used for any further discussion in the remainder of the article. 12231
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Table 7. Structural Parameters for CHClF2 3 3 3 HCCH Derived by Fitting Ia and Ib from All Four Isotopologues to R||| 3 3 3 C, ϕC‑||| 3 3 3 C and θClC 3 3 3 |||, with the Structures of the Monomers Fixeda original (ref 1) R||| 3 3 3 C (Å) ϕC||| 3 3 3 C (deg)
θClC 3 3 3 ||| (deg) R||| 3 3 3 H (Å) RCl 3 3 3 H (Å) θCH 3 3 3 ||| (deg) θCl 3 3 3 HC (deg) σ (u Å2)c
Figure 3. Significant structural parameters obtained from an inertial fit structure of CHBrF2 3 3 3 HCCH using the Ia and Ib moments from all observed isotopologues. The three parameters that were fitted are given at the bottom of the figure.
Table 6. Absolute Values of Principal Axis Coordinates (Å) for the Bromine Atom in CHBrF2 3 3 3 HCCH (Determined Using Kraitchman’s Equations) and a Comparison with Inertial Fit and ab Initio Derived Coordinates a
b
c
0.8061(19) 0.8157(18)
0.8471(18) 0.8416(18)
0.046(33)i 0.053(29)i
0.8285
0.8324
0.0000
ab initio structure I
0.8612
0.8149
0.0000
ab initio structure II
0.9744
0.7528
0.0555
ab initio structure III
1.5613
0.2930
0.0000
ab initio structure IV
0.1888
0.0022
0.1318
79
12
12
CH BrF2 3 3 3 H C CH parent CH79BrF2 3 3 3 H13C13CH parent a inertial Fit b
a
The derived a and b coordinates for the 13C2H2 species were transformed to the principal axis system of CH79BrF2 3 3 3 12C2H2 to allow for direct comparison. The imaginary value of the c coordinate was not transformed. b Coordinates tabulated for the inertial fit were taken from the fit of the lowest standard deviation (Table 5; using Ia and Ib).
Since we have two sets of isotopic data in which only the mass of the Br atom is changed, we can obtain two independent determinations of this atom’s coordinates via Kraitchman’s equations.22,23 CH79BrF2 3 3 3 H12C12CH and CH79BrF2 3 3 3 H13C13CH isotopologues can be used as the parent species, allowing two separate calculations of the bromine atom principal axis coordinates. Coordinates derived using the 13C2H2 isotopologue as parent have been transformed into the principal axis system of CH79BrF2 3 3 3 12C2H2 to allow direct comparison with parameters from ab initio structures and the inertial fit. These results are summarized in Table 6, where it can be seen that good agreement is achieved both with the inertial fit and structure I of the ab initio optimizations. Values of the off-diagonal component of the nuclear quadrupole tensor (χab) can provide some additional structural information via the determination of the angle between the z axis of the quadrupole tensor (assumed to lie along the CBr bond) and the a inertial axis of the dimer using eq 2 2χab ¼ ðχaa χbb Þ tan 2θza
ð2Þ
This yields an angle θza of 69.37(1)° for the CH BrF2 3 3 3 H12C12CH species, in reasonable agreement with an ab initio 79
rs
MP2
ref 4b
3.7101(40)
3.7063(45)
3.7132(49)
3.6762
69.1(13)
71.1(16)
79.3(24)
68.4
88.0(5)
88.3(5)
88.6(6)
93.2
2.730(6)
2.724(7)
2.730(7)
2.655
3.061(38)
3.122(46)
3.362(69)
3.166
148.2(6)
148.7(7)
150.8(8)
157.0
114.4(16) 0.053
112.0(19) 0.061
102.7(26) 0.070d
115.2
The original structure is the one reported in ref 1, fixing the CHClF2 monomer at the r0 parameters given in ref 25. The rs structure fixes the CHClF2 monomer at the rs parameters given in ref 25. The MP2 structure fixes the CHClF2 monomer at the MP2/aug-cc-pCVQZ calculated structure reported in ref 4. For all fits, the HCCH structure is that given in Table 4 in the present article. See text for a discussion of the relative merits of the different CHClF2 3 3 3 HCCH structures; the rs structure is favored. b The structure in ref 4 was obtained by manually adjusting the intermolecular parameters to reproduce the rotational constants of CH35ClF2 3 3 3 H12C12CH. Uncertainties are not available for that fit. c Standard deviation of the fit. d A fit using only the moments of inertia of CH35ClF2 3 3 3 H12C12CH gives a standard deviation of 0.280 u Å2, with structural parameters nearly the same as those obtained in ref 4. a
value of 65.9° and an inertial fit value of 68.4(5)°. Values of θza determined from χab for the other isotopic species are all in close agreement, ranging from 69.4° to 70.0° (Table 3).
IV. DISCUSSION The CHBrF2 3 3 3 HCCH complex structure clearly exhibits a CH 3 3 3 π interaction, as expected based on the most stable geometry (I) derived from ab initio calculations (Figure 1) and by comparison to the chlorine analogue CHClF2 3 3 3 HCCH.1 The CH 3 3 3 π distance is found to be 2.670(8) Å, close to our original value of 2.730(6) Å for CHClF2 3 3 3 HCCH,1 although a refitting of the complex structure by Vincent and Hillier using an MP2 calculated CHClF2 monomer4 put the CH 3 3 3 π distance at 2.655 Å, almost exactly the same as in the present case. This discrepancy prompted further extensive investigations by us (described below) of the CHClF2 3 3 3 HCCH dimer structure using our previous experimental data,1 and these consistently give H 3 3 3 π distances in the 2.722.73 Å range. Attempts at using a least-squares fitting procedure to reproduce the structure of CHClF2 3 3 3 HCCH reported by Vincent and Hillier,4 using their ab initio CHClF2 monomer and our experimental rotational constants (with both Schwendeman’s STRFITQ program21 and Kisiel’s STRFIT program24), were only partially successful. Their structure reproduces the rotational constants of the most abundant isotopologue well but is less successful at reproducing constants for other isotopologues. Most of the final structural parameters from our fit using their MP2 CHClF2 monomer geometry agree reasonably well with those we originally reported in ref 1 (using Cramb’s r0 monomer structure25), although parameters relating to the tilt of the acetylene subunit with respect to the intermolecular axis showed a larger variation. Using the rs structure of Cramb25 for CHClF2 gives results similar to those of our refit using the MP2 monomer, 12232
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but with a lower standard deviation for the fit, and it reproduces the monomer planar moment much better than our original fit,1 our refit using the MP2 monomer,4 or Hillier’s refit.4 The original dimer structure that we reported in ref 1 as well as the refitted structures using the rs and MP2 monomers (and the structure suggested in ref 4) are all summarized in Table 7. We favor the CHClF2 3 3 3 HCCH configuration that uses the rs monomer of Cramb25 because it reproduces the observed Pcc best and has the lowest standard deviation of the refitted structures. It is clear that the largest differences between structures are in the Cl 3 3 3 H distance and C 3 3 3 |||C and Cl 3 3 3 HC angles. Similar concerns arise for the current complex, where the existing structure of CHBrF2 fixes some of the structural parameters (specifically the CH distance and HCBr angle) at the values obtained from the rs structure for CHClF2 (these parameters could still not be precisely determined, even after addition of the 13CHBrF2 data discussed in section III.B.);13 however, the observed and calculated planar moments for CHBrF2 3 3 3 HCCH are in perfect agreement, giving us additional confidence in our fitted structure for this dimer. Comparison of the contact distances between the nearest acetylenic hydrogen atom and the Br atom (3.293(40) Å in CHBrF2 3 3 3 HCCH) or Cl atom (3.12(5) Å using the CHClF2 3 3 3 HCCH structure with the rs monomer) shows an increase of about 0.17 Å going from the Cl to Br analogue of the halomethane, very close to the 0.15 Å difference in the van der Waals radii of Cl and Br (1.8 Å and 1.95 Å).26 The distance between the center of the triple bond and the halomethane C atom is also worthy of brief mention: it is similar (3.6/3.7 Å) in all four halomethane/HCCH complexes studied so far (CHBrF2 = 3.683(7) Å; CHClF2 = 3.706(5) Å; CH2ClF = 3.605(4) Å; and CH2F2 = 3.625(9) Å) whether the hydrogen atoms of the halomethane straddle the triple bond (as in CH2F2 3 3 3 HCCH3 and CH2ClF 3 3 3 HCCH2) or lie in the symmetry plane (as in CHBrF2 3 3 3 HCCH and CHClF2 3 3 3 HCCH1). The trend appears to be reversed when examining H 3 3 3 π distances, with the distance in CHBrF2 3 3 3 HCCH (2.670(8) Å) about 0.05 Å shorter than that in CHClF2 3 3 3 HCCH (2.724(7) Å). In the complexes that contain double CH 3 3 3 π interactions (CH2ClF 3 3 3 HCCH and CH2F2 3 3 3 HCCH), the distances are longer (3.236(6) Å and ∼3.36(1) Å, respectively) due to the extension of the CH bonds out of the symmetry plane. In addition to the structural data, the intermolecular binding energy in CHBrF2 3 3 3 HCCH can be estimated using the pseudo diatomic approximation27,28 (eq 3) EB ¼
1 ks Rcm 2 72
ð3Þ
where the intermolecular stretching force constant for the weak interaction, ks, is obtained from derived spectroscopic and structural parameters, including the centrifugal distortion constant, DJ, by eq 4 as follows: 16π4 ðμRcm Þ2 ½4B4 þ 4C4 ðB CÞ2 ðB þ CÞ2 ks ¼ hDJ
CH2ClF 3 3 3 HCCH complexes1,2 (EB = 4.9(5) and 4.75(4) kJ/mol, respectively), while preliminary results for CH2F2 3 3 3 HCCH indicate a binding energy of 3.88(6) kJ/mol.3 There is no clear correlation between the binding energy and dipole moment of the monomer. The CHBrF2 and CHClF2 monomer dipole moments are very similar (μtotal = 1.50 and 1.42 D),29 yet these are determined to be the most weakly and most strongly bound complexes, respectively; the acetylene complexes containing the more polar CH2F2 (μtotal = 1.97 D) and CH2ClF (μtotal = 1.82 D) monomers are, however, still among the more strongly bound dimers. Since all components of the nuclear quadrupole tensor in the principal axis frame (χaa, χbb, χcc, and χab) have been determined, it is possible to recover the χxx, χyy, and χzz components by diagonalization. For the CH79BrF2 3 3 3 HCCH species, the values obtained are χxx = 275.317(9) MHz, χyy = 275.310(15) MHz, and χzz = 550.626(17) MHz. These are in close agreement with values obtained for the CHBrF2 monomer (χxx = 284.9(23) MHz, χyy = 279.80(6) MHz, and χzz = 564.7(23) MHz),13 indicating that perturbation of the electric field gradient at the Br atom upon complexation is minimal. Interestingly, the χxx and χyy components in the present dimer are identical, indicating that cylindrical symmetry exists about the CBr bond.
V. CONCLUSIONS CP-FTMW and resonant cavity FTMW measurements of the rotational spectra for four isotopologues have provided a structure for the CHBrF2 3 3 3 HCCH complex. This structure has Cs symmetry exhibiting a weak CH 3 3 3 π interaction and agrees with the most stable structure obtained from ab initio optimizations. The observed CH 3 3 3 π distance in this complex is close to that in CHClF2 3 3 3 HCCH, and the contact distance between the Cl or Br atom and the closest H atom of HCCH differs by 0.17 Å, almost the same as the difference in the van der Waals radii of Cl and Br (0.15 Å). Additional theoretical work is in progress to analyze the driving forces behind the structural preferences and relative contributions to the interaction energy in these and related complexes and will be published separately. ’ ASSOCIATED CONTENT
bS
Supporting Information. Measured rotational transition frequencies for (i) 13CH79BrF2 and 13C81BrF2 monomers; (ii) CH79BrF2 3 3 3 H12C12CH, (iii) CH81BrF2 3 3 3 H12C12CH, (iv) CH79BrF2 3 3 3 H13C13CH, and (v) CH81BrF2 3 3 3 H13C13CH dimers; and (vi) principal axis coordinates for the inertial fit structure and ab initio structure of CHBrF2 3 3 3 HCCH. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (217) 581-2679. E-mail:
[email protected]. ð4Þ
where μ is the reduced mass of the complex, Rcm is the center of mass separation, and all quantities are used in base SI units. This gives an interaction energy of EB = 2.46(3) kJ/mol and a force constant of ks = 1.82(1) N/m; thus, this complex is estimated to be less strongly bound than the CHClF2 3 3 3 HCCH and
’ ACKNOWLEDGMENT This work was supported by the National Science Foundation (RUI CHE-0809387 at Eastern Illinois University and MRI-R2 CHE-0960074 at the University of Virginia). R.A.P. and S.A.P. also gratefully acknowledge Professor Robert Kuczkowski for the provision of the 13C2H2 sample. 12233
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