The rotational spectrum and molecular structure of the benzene–hydrogen chloride complex W. G. Read, E. J. Campbell, and Giles Henderson Citation: The Journal of Chemical Physics 78, 3501 (1983); doi: 10.1063/1.445173 View online: http://dx.doi.org/10.1063/1.445173 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/78/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The tetrahydrofuranhydrogen chloride complex: Rotational spectrum and theoretical analysis J. Chem. Phys. 111, 6363 (1999); 10.1063/1.479962 Benzene-hydrogen halide interactions: Theoretical studies of binding energies, vibrational frequencies, and equilibrium structures J. Chem. Phys. 108, 7217 (1998); 10.1063/1.476139 The rotational spectrum and molecular structure of the argon–carbonyl fluoride complex J. Chem. Phys. 79, 4724 (1983); 10.1063/1.445614 The rotational spectrum and molecular properties of the hydrogen cyanide hydrogen bromide complex J. Chem. Phys. 78, 3494 (1983); 10.1063/1.445172 The rotational spectrum and molecular properties of a hydrogenbonded complex formed between hydrogen cyanide and hydrogen chloride J. Chem. Phys. 76, 2267 (1982); 10.1063/1.443299 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.63.180.147 On: Wed, 13 Aug 2014 15:08:26
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The rotational spectrum and molecular structure of the benzene–hydrogen chloridecomplexW. G. Read, E. J. Campbell, and Giles Henderson
Citation: The Journal of Chemical Physics 78, 3501 (1983); doi: 10.1063/1.445173 View online: http://dx.doi.org/10.1063/1.445173 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/78/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The tetrahydrofuranhydrogen chloride complex: Rotational spectrum and theoretical analysis J. Chem. Phys. 111, 6363 (1999); 10.1063/1.479962 Benzene-hydrogen halide interactions: Theoretical studies of binding energies, vibrational frequencies, andequilibrium structures J. Chem. Phys. 108, 7217 (1998); 10.1063/1.476139 The rotational spectrum and molecular structure of the argon–carbonyl fluoride complex J. Chem. Phys. 79, 4724 (1983); 10.1063/1.445614 The rotational spectrum and molecular properties of the hydrogen cyanide hydrogen bromide complex J. Chem. Phys. 78, 3494 (1983); 10.1063/1.445172 The rotational spectrum and molecular properties of a hydrogenbonded complex formed between hydrogencyanide and hydrogen chloride J. Chem. Phys. 76, 2267 (1982); 10.1063/1.443299
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The rotational spectrum and molecular structure of the benzene-hydrogen chloride complex
w. G. Read, E. J. Campbell, and Giles Hendersona)
Noyes Chemical Laboratory, University of Illinois. Urbona, Illinois 61801 (Received 20 July 1982; accepted 2 August 1982)
The microwave spectrum of the weakly bound complex benzen~HCI was studied in the gas phase using Fourier transform microwave spectroscopy carried out in a Fabry-Perot cavity with a pulsed supersonic nozzle as the molecular source. Several R -branch a -dipole transitions have been observed for benzen~H "CI, benzen~D "CI, benzen~H "CI, and benzene(d6l-H "C!. The spectrum was characteristic of a symmetric top, indicating that the time averaged displacement of the Hand CI atoms from the benzene C 6 axis is zero. Deuterium substitution of He I demonstrated that the acidic proton lies between the CI atom and the benzene ring. The chlorine nuclear quadrupole coupling constant X;:, was measured for all four isotopic species and is interpreted in terms of a projection of the chlorine quadrupole coupling constant in free HCI, averaged over two degenerate vibrational ground state bending modes involving the angles between the benzene C 6 axis and the HCI bond axis. The spectroscopic constants for benzen~HCI are:
Isotope Bo (MHz)a D~ (kHz)
Benzene-H 35CI 1237.6836(5) 1. 22(1)
Benzene-H 31CI 1201. 925(2) 1.16(4)
Benzene - D 35C I 1228.2440(6) 1.19(2)
Benzene(ds)-H 3'CI 1165.1542(6) 1. 08(1)
'Numbers in parentheses represent one standard deviation in the fit.
I. INTRODUCTION
Weak hydrogen-bonded complexes formed by the attractive interaction between the electrophilic hydrogen halides and the 1T-electron densities of unsaturated hydrocarbons have been of great importance as intermediates in both addition and electrophilic substitution reactions. 1- 5 Cryogenic freezing point studies have revealed that HCl forms weak 1:1 and 2:1 complexes with alkenes, 1:1, 2:1, and 4:1 complexes with alkynes, and 1:1 complexes with benzenes. Although numerous IR, Raman, and NMR studies have been made on such hydrogen bonded 1T-electron systems, 7_1S these measurements have all been on condensed phases, in which the properties of the species of interest are perturbed by lattice or solvent effects.
This laboratory has reported a very sensitive high resolution Fourier transform microwave technique that allows the measurement of rotational spectra of weak molecular complexes in a pulsed supersonic expansion of high pressure gas mixtures. 17,18 Spectra obtained in this manner have provided detailed structural information on several gas phase hydrocarbon-hydrogen halide complexes, including cyclopropane-HCl, 19 cyclopropane_HF,20 ethylene-HCl,21 ethylene-HF, 22 acetyleneHCl,23 acetylene-HF, 24 and more recently we have reported the first microwave observation and identification of benzene-HCI. 25
In this report we present our spectroscopic measure-
a) Permanent address: Department of Chemistry, Eastern Illinois University, Charleston, Illinois 61920.
D~K (kHz) X~,.l (MHz)
13.35(2) -52.19(2)
13.4(8) -41.2(3)
14.57(3) -54.70(2)
10.93(4) - 52. 25(2)
ments for benzene-H35Cl, benzene-H37Cl, benzeneD35Cl, and a more recently studied isotopic species, benzene(ds)-H35Cl, which was not included in that earlier work. 25 Certain structural and dynamic properties of benzene-HCl obtained from the vibrational ground state microwave spectrum will be discussed. The vibrationally averaged structure will be shown to be a symmetric top possessing Csv symmetry, with the time averaged positions of Cl and H situated on the C6 axis of benzene, rather than HCI being located above one of the C-C bonds of benzene. Our interpretation of the chlorine nuclear quadrupole coupling indicates that HCl undergoes large amplitude bending motions in the ground vibrational state. The strength of the benzene-HCl bond is compared to those of previously studied complexes and is found to be similar to hydrogen bonded complexes involving HCl with hydrocarbons but is much stronger than rare gas-hydrogen halide van der Waals molecules.
II. EXPERIMENTAL
The Fourier transform spectrometer used in this experiment has been described elsewhere. 17•18 Our gas mixtures consisted of full vapor pressure (2-3 in.) of benzene (Fisher SCientifiC, certified A. C. S.) or benzene(dg) (Merck, 99.6 atm%D), and 1-2 in. of Hel or DCl (MSD Isotopes, !:I !:I. 5% DJ, which was pressurized to 20 PSI with argon. The benzene-HCl complexes are made by pulsing the above mixtures initially at room temperature into an evacuated Fabry-Perot cavity through a O. 5 mm diameter orifice. As a result of the gas expansion only the ground vibrational state is sufficiently populated to allow the observation of micro-
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3502 Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex
TABLE I. Observed and calculated rotational transition frequencies for benzene-H3·Cl.
wave transitions. The observation of a J = 4 - 5, K = 4 transition indicates that complexes with rotational temperatures as high as 16 K exist in sufficient quantities for measurement. Rotational superposition states of benzene-HCI are prepared by a pulsed microwave standing wave adjusted in power and duration to produce maximum sample polarization. 2&,27 The coherent free induction decay from all the rotational transition frequencies within the 1 MHz bandwidth of the cavity is then detected and amplified with a gated superheterodyne receiver. The time domain signal is digitized, averaged, and Fourier transformed to give the frequency domain power spectrum.
III. RESULTS
The transition frequencies for four isotopic species of benzene-HCI observed in their vibrational ground state are listed along with their assignments in Tables I-IV. These spectra clearly correspond to a centri-
fugaUy distorted symmetric top exhibiting fully resolved nuclear quadrupole hyperfine structure resulting from a spin 3/2 nucleus. The Hamiltonian used to fit the data may be written28
H = Hg + Q(CI) : V(Cl) , (1)
where Hg is the rotational Hamiltonian which includes the quartic distortion constants D. and DTX • Q(Cl) is the 35CI or 37CI nuclear quadrupole moment tensor, and V(Ci) is the electric field gradient tensor at the chlorine nuclear site. The deuterium quadrupole coupling constants in benzene-DCI and benzene(d~-HCI have been neglected, since the corresponding hyperfine structure was unresolved in these spectra (splittings of 30 kHz or less). For a symmetric top molecule containing a single chlorine atom on the a-inertial axiS, the quadrupole interaction is characterized by the coupling constant Xaa = I e I Q(CI) Vaa, where e is the fundamental charge, Q(Cl) is nuclear quadrupole moment of chlorine,
J. Chern. Phys., Vol. 78, Part II, No.6, 15 March 1983
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Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex 3503
TABLE n. Observed and calculated rotational transition frequencies for benzene-H37Cl.
Assignment Observed J K F J' K' F' frequency (MHz)
and Vaa is the electric field gradient measured at the chlorine nucleus in the direction of the a inertial axis. The matrix elements for the quadrupole interaction of the Hamiltonian Eq. (1) in the coupling scheme Iel + J = F have been derived before. 29 A least squares fit of all the measured tranSitions of a given isotopic species to those calculated by an exact diagonalization of the Hamiltonian matrix was carried out to obtain the spectroscopic constants Bo, D1 , D1K, and X~.: summarized in Table V. The observed and calculated frequencies are in good agreement, with typical reSiduals of a few kHz, as can be seen in Tables I-N.
The symmetric top spectrum observed here indicates that the time-averaged displacement of the Hand CI atoms from the benzene Cs axis is zero. For a slightly asymmetric top, those pairs of transitions characterized by the prolate symmetric top quantum numbers K = + 1 and K = - 1 would be separated by (J + 1) (B - C) for the J - J + 1 transition. 28 Because these transitions are observed to occur at exactly the same frequency, within the limits of error in our experiment of perhaps 30 kHz
TABLE m. Observed and calculated rotational transition frequencies for benzene-D35Cl.
Assignment Observed Calculated Difference
J K F - J' K' F' frequency (kHz) frequency (MHz) (kHz)
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3504 Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex
TABLE IV. Observed and calculated rotational transition frequencies for benzene (ds)-H35CI.
Assignment Observed Calculated Difference
J K F J' K' F' frequency (MHz) frequency (MHz) (kHz)
for the J = 4 - 5 tranSition in benzene-H35CI, our results set an upper bound on B - C of approximately 6 kHz. A structure with the HCI located above the benzene plane but directly over a carbon-carbon bond would yield B - C of 23 MHz. A planar structure analogous to cyclopropane-HCl19 would give B - C of 270 MHz.
We will assume that the ground state vibrationally averaged structures for HCI and benzene, "given in Table VI, " remain unchanged after complexation and describe the structure of benzene-HCI in terms of the parameters depicted in Fig. 1. The center of mass separation of the two subunits is given by Ro, and the instantaneous orientation of HCI, with respect to the principal axes of the benzene subunit is given by e and
the benzene-HCI structure parametrized by the three coordinates Ro, e, and cp over the cp coordinate, assuming that the cp dependence in the ground state wave function has Ca symmetry, we obtain for the moment of inertia of benzene -HCI
where IpD is a pseudodiatomic moment, IJ.PDR~ in which IJ.PD = maZmKC/(mBZ +mHCl)' m denotes mass, and lbb(benzene) and IKCl are the appropriate moments of inertia of free benzene and HCI, respectively.
cp. We will assume that since benzene has Ca symmetry, the cp part of the potential will also have Ca symmetry. By averaging the instantaneous moments of inertia for
A symmetric top spectrum is consistent with two possible orientations of HCI with respect to the benzene plane, one which locates the acidic proton between the CI and the ring and one which locates this hydrogen on the opposite side of CI, making it the furthest atom from the ring. For any given choice of 8'; we may use
TABLE V. Spectroscopic constants for the benzene-hydrogen chloride complex.
Isotope
benzene-H35CI
benzene-H 37CI
benzene-D 35Cl
benzene(ds) H 35CI
Bo (MHz)
1237.68362(52)
1201.92498(170)
1228.24396(58)
1165.15416(61)
D" (kHz) D"K (kHz) X;"l (MHz)
1.223(13) 13.35 (2) - 52.189 (18)
1.155 (39) 13.41 (81) - 41. 248 (275)
1.186 (15) 14.57 (3) - 54. 705 (22)
1.082 (14) 10.93 (4) - 52. 254 (19)
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Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex 3505
TABLE VI. Spectroscopic and molecular constants of free benzene and HCI.
benzene and benzene (de)
ro (C-C) (A)a 1. 396 4(2)
ro (C-H) (At 1. 0831(13)
!fOCI H3TCI }j°CI
Bo (MHz)b 312989.297 312519.121 161656.238
ro (A)" 1. 28387 1. 283 86 1. 28124
Xo (MHz)d -67.61893 - 53. 294 -67.39338
aReference 34. "Calculated from Bo. ~eference 35. dReference 36.
Eq. (2) to obtain a value of Ro that reproduces the measured B rotational constant for that particular isotopic species. Table vn gives a comparison among the four isotopic species of the center of mass to chlorine distances R for selected values of the angle 9. It can be seen that the most consistent agreement among the R values for the different isotopes is obtained when the hydrogen of HCI is located between chlorine and benzene.
Although Table VII may suggest that a value of 9"" 20° best reproduces the rotational constants, more direct information on the HCI zero point bending motion can be obtained from the chlorine nuclear quadrupole coupling constant x;i, which is related to Xo in free HCI through a vibrationally averaged projection:
x;i = txo{3 cos2 9 -1) • (3)
Equation (3) neglects any long range electrical effects due to the presence of benzene. Such effects, which include external field gradients and bond polarization of HCI due to the multipole moments of benzene, as well as bond elongation of HCI, have been discussed elsewhere. 22 ,30 The above effects are expected to be small and will be neglected in this treatment. Values for 9* = arc cos(2x;i /3Xo + 1/3)1/2 corresponding to the vibrationally averages P2(COS 9) projection function of Eq. (3) that fit the observed nuclear quadrupole coupling constants are given in Table VIII for each isotopic
RO
center of moss
a
FIG. 1. Intermolecular coordinate system and identification of the inertial axes (abc) in benzene-hydrogen chloride. The distance Ro used in the text separates the centers of mass of the two subunits. The average displacement of the H and Cl atoms from the benzene C6 axis is zero.
species. The spectroscopic data is consistent with a value of 9*"" 23°, with the exception of benzene DCI, where the average zero point displacement of the heavier deuterium atom is of smaller amplitude, giving 9* = 21°. It should be noted that the angles obtained from Eq. (3) cannot be expected to agree exactly with those obtained from Eq. (2), in part because the reduced amplitude of the DCI bending is a real effect, but also because these two equations use expectation values of two different functional. forms involving 9.
B. Equilibrium structure
There are two possible equilibrium structures consistent with the previously described vibrationaUy averaged structure: (1) the HCI subunit is coaxial with the Cs axis of benzene, or (2) the HCI center of mass is located on the benzene Cs axis but the HCI axis is tilted at some angle 9. ¢Oo, with the hydrogen atom presumably directed towards the edge of the benzene ring. An
TABLE VII. Comparison of benzene center-of-mass-to-chlorine distance R for selected values of the HCr bending angle 8Rel • a
aValues for BzW5Cl and Bzn35CI in Table I of Ref. 25 are different from those presented here because the benzene structure used in Ref. 25 was obtained from an earlier study by B. P. Stoicheff, Can. J. Phys. 32, 339 (1954).
lrel position is taken as the projection of the HCl center-of-mass-to-Cl distance onto the benzene C, axis. "u is the standard deviation of R for the set of isotropic species at a given 8Rel •
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3506 Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex
TABLE VIII. The vibratlonally aver-aged structure of benzene HCl. a
Isotope lIib (deg) Ro (A)
Bz~5Cl 22.96(2) 3.5938
BzH3TCl 22.84(30) 3.5952
BzIY5Cl 20.74(2) 3.5697
Bz(ds)H35Cl 22.90(2) 3.5910
asee Fig. 1 and the text for definitions of these quantities.
equilibrium structure of the first type would correspond to a potential surface with a single minimum at 9. = 0°. ill contrast, an equilibrium structure of the second type would correspond to a potential surface with a minimum trough at (Ie '" 0 0 with varying depths in the cP direction reflecting the Cs symmetry of benzene, and a relatively low barrier at 9 = 0°, permitting tunneling at frequencies which are high compared to molecular rotation frequencies. Both cases give vibrationally averaged rotational constants characteristic of a prolate symmetric top with the moment of inertia about the bprinCiple axis consistent with Eq. (2). If the 9 barrier height of the type (2) potential surface is increased, the ground state tunneling period becomes long compared to the period of molecular rotation and the exact K degeneracy of the symmetric top is removed, resulting in an asymmetric top spectrum. Anisotropies in the cP part of the type (2) potential surface would also introduce splittings in the spectrum due to hindered internal rotation of the hydrogen atom relative to the benzene frame as observed in the microwave spectrum of the , u methyl acetylene-HF complex. The observed spec-trum provides no evidence for the types of splittings that would indicate a slight asymmetry or hindered internal rotation in benzene-Hel. Moreover, the projection angle of the chlorine nuclear quadrupole coupling constant clearly decreases upon deuteration of Hel (see Table VII]). This behavior is consistent with the type (1) CSv equilibrium structure for which an increase in the moment of inertia of Hel will result in a decrease in the Hel zero point bending energy and a more sharply peaked wave function, giving an increase in the vibrationally averaged projection of Xo. Although the low effective temperature of our beam precludes the observation of vibrationally excited states, the observed ground
state spectra strongly suggest the type (1) C6v equilibrium geometry in which all the delocalized 1T electrons contribute equally to the bonding interaction.
v. INTERMOLECULAR FORCES
The susceptibility of the hydrogen bond to centrifugal distortion provides semiquantitative information regarding the radial interaction between benzene and Hel. It is also possible to estimate a bending force constant for Hel using the bending angle amplitudes in Table VIII. Here we develop a relationship between DJ and the radial force constant ka, which retains the simple form of a pseudodiatomic model of D}T but properly includes the inertial properties of the subunits. DJ is defined as32
.r..~) is the partial derivative of the a" component of the moment of inertia tensor with respect to the ith internal coordinate, evaluated at the equilibrium structure. Ij} is the i-jth element of the inverse force constant matrix, and I"" is the equilibrium moment of inertia about the ath axis, although the ground state I"" will be used here. We assume that only hydrogen bond vibrations contribute to DJ since these modes will have the largest inverse force constants. We take a model complex conSisting of rigid rod and a rigid body in which the rod is colin ear to one of the inertial axes of the body in the equilibrium position. This condition applies to the vast majority of molecules studied in our laboratory. The system has at most five vibrational degrees of freedom which include a stretch along the centers of mass of the two subunits, the two bending motions for the rod, and as many as two bending motions for the body. All motions are assumed to be harmonic. Expanding Eq. (5) using the derivatives appropriate for the assumed equilibrium structure, and substituting the results into Eq. (4), we find that DJ depends only on the stretching force constant as
81T3(lJ.poRo)2 [(B2 + C2)2 + 2(B4 + C4)] DJ =
Fika (6)
TABLE IX. Approximate intermolecular force constants and potential parameters for various complexes of HCl.
k. ~ k. t: ks Molecule (mdynekl)a (cm-I)a (mdyne A-I)b (cm-I)b (mciyneA)
aFrom pseudodiatomic model for D J • b(;sing Eq. (6) or Eq. (7).
J. Chern. Phys., Vol. 78, Part II, No.6, 15 March 1983
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Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex 3507
TABLE X. Well depths and bond lengths for several hydrocarbon HCI complexes. (Bond lengths are center of mass of hydrocarbon to CI distance except for cyclopropane-HCI which is midpoint of C-C bond to CI distance.)
Complex Well depth (cm-l )& Bond distance (A)
Cyclopropane-HCI 855 3.57b
Benzene-HCI 720 3.63
Acetylene-HCI 614 3.69c
Ethylene-HCI 575 3.7Z'
&computed using force constants obtained from Eq. (6) or Eq. (7).
"see Ref. 19. cSee Ref. 21. c:\See Ref. 22.
For a symmetric top Eq. (6) reduces to
D~= 64w3(lLpDRO)2 B'j1ika • (7)
In Table IX the force constants and well depths are listed for a series of different HCI complexes using Eq. (6) or Eq. (7) and the pseudodiatomic model17 for D~. It is interesting to note that the pseudodiatomic treatment gives satisfactory agreement with the results of Eq. (6) or Eq. (7) in those cases where the moments of inertia of the subunits are small. Once ka is obtained, a standard procedure is used to estimate the well depth, €, for benzene-HCI assuming a Lennard-Jones 6-12 potential. This procedure is described in detail elsewhere17 and the results are listed in Table IX.
H we treat the HCI as a harmonic hender in an isotropic two-dimensional well, we obtain
(82)2 = 1i2 JIBe I k, , (8)
relating the expectation value of 82 to the harmonic force constant k, for the bending motion. H we approximate (82) with (8*)2, using 8* = 22. 96° for benzene-H35CI and the moments of inertia of H35CI and D35CI, this harmonic model predicts 8* = 19.5° for benzene- D35CI, compared to the measured value of 20.7°. We note that 8* for benzene (ds)-H35CI (22.90°) is nearly equal to 8* for benzene-H35CI (22. 96"), suggesting that benzene motions are not strongly coupled to the HCI bending motion. Using Eq. (8) we obtain the value for k, listed in Table IX. Only the value for benzene-H35CI is given as these numbers vary by no more than 10% among the four isotropic species studied.
VII. DISCUSSION
Measurements of the vibrational ground state, rotational spectra of four isotopic species of benzene-HCI indicate that this molecule is a CSv symmetric top, with the HCI located above the plane of the benzene ring and with the acid proton lying between benzene and the CI atom. The time averaged displacement of the Hand CI atoms from the benzene Cs axis is zero. Although the equilibrium structure of benzene-HCI is likely to have the HCI located on the benzene Cg axiS, this result cannot be properly established until excited vibrational states are studied. The measured hydrogen bond well
depths estimated from centrifugal distortion data are comparable in magnitude to those obtained previously for cyclopropane-HCI,19 acetylene-HCI,23 and ethyleneHCl. 21 The operationally defined bending angle 9* between the HCI figure axis and the Cs axis of benzene is 2° larger than corresponding angles in the other hydrocarbon-HCI complexes. This is probably due to the delocalization of the w-electron denSity in benzene resulting in a broad potential surface for the bending motion.
In Table X we have listed well depths and bond lengths for several HCI complexes. It is interesting to note the inverse correlation between bond length and bond strength for HCI complexes with hydrocarbons. In order of increasing well depth and decreasing bond length, we have ethylene-HCI, acetylene-HCI, benzene-HCI, and cyclopropane-HCI.. Indeed this series is chemically intuitive, so we would expect benzene-HCI, with six w electrons, to be more tightly bound than acetyleneHCI and ethylene HCI, which have four and two w electrons, respectively. The short C-C to CI bond length for cyclopropane-HCI is reasonable since cyclopropane is a strained ring system with a Significant amount of charge density extending beyond the triangle defined by the three carbon atoms.
VII. ACKNOWLEDGMENT
The support of the National Science Foundation is gratefully acknowledged.
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3508 Read, Campbell, and Henderson: Spectrum of the benzene-hydrogen chloride complex
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