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High Resolution Millimeter-Wave Spectroscopy of Cyclopropylphosphine−Borane Roman A. Motiyenko,†,* Laurent Margulès,† and Jean-Claude Guillemin‡ †

Laboratoire de Physique des Lasers, Atomes et Molécules, UMR CNRS 8523, Université de Lille 1, F-59655 Villeneuve d’Ascq, France ‡ ́ Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France S Supporting Information *

ABSTRACT: The microwave spectrum of cyclopropylphosphine−borane, C3H5PH2−BH3, has been investigated in the frequency range 150−195 GHz. The spectral assignment was supported by high level ab initio calculations. Two stable conformations have been predicted: the most stable antiperiplanar form and synclinal form that is higher in energy by 7.3 kJ/mol. In the observed spectra, only the most stable antiperiplanar (ap) form has been assigned. The analysis of the rotational spectra in the lowest excited vibrational states of the ap conformer has enabled determination of the potential function for the C−P torsional mode in the vicinity of equilibrium position. The barrier to internal rotation of the BH3 top has been determined to be 9.616(15) kJ/mol and agrees well with quantum chemical calculations.

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INTRODUCTION In a recent study, Németh et al.1 have presented a thorough analysis of several amine− and phosphine−boranes by means of NMR and photoelectron spectroscopy, mass spectrometry, and augmented by quantum chemical calculations. Growing interest in such compounds as potential hydrogen storage materials motivated the study presented in ref 1, and this property was examined by theoretical methods. On the other hand, the recent recording of the infrared2 and photoelectron spectra of the vinylphosphine−borane3 showed how the nature of the phosphine is correlated to the kinetic stability of the donor−acceptor complex. Microwave spectra of primary phosphine−boranes are scarce, whereas to our knowledge only the high-resolution spectra of methyl and ethyl phosphine−boranes have been reported up to date.4,5 By the peculiar properties of the cyclopropyl group between those of an alkyl, vinyl, and allyl substituent,1 extending the spectroscopic studies on the cyclopropylphosphine−borane (C3H5PH2−BH3, hereafter CPPB) presents a great interest. Rotational spectroscopy providing high resolution and accuracy is an appropriate tool for exploration of molecular conformational equilibrium and potential energy surface. Our study was also motivated by determination of the barrier to hindered rotation of the BH3 top relative to molecular frame. Compared to the methyl group, only few data are available for internal rotation of boranes. For example, in the case of H3N−BH3, the barrier height was determined to be 8.565(38) kJ/mol,6 whereas for C2H5N−BH3, the barrier height of 12.1(3) kJ/mol has been reported.7 © 2012 American Chemical Society

As it was found by Nemeth et al., on the basis of quantum chemical study, two stable conformations may exist for C3H5PH2−BH3. Their rotational isomerism is shown in Figure 1. The antiperiplanar conformer (denoted hereafter ap), having the H−C−P−B dihedral angle of 180°, was found to be the most stable in ref 1. The synclinal conformer (denoted hereafter sc) is obtained from the ap by rotation about the C−P bond by 129°. Because the direction of the rotation can be either positive or negative, there are two equivalent configurations of the sc conformation with opposite sign of the H−C−P−B dihedral angle. In ref 1 this conformer was found to be less stable than the ap by 5.1 kJ/mol. The quantum chemical calculations presented in ref 1 have been performed at a high level of theory and provided accurate values of spectroscopic constants for C3H5PH2−BH3 and exploration of potential energy function for the torsion along C−P bond. However, some additional information on dipole moment components, centrifugal distortion constants, low frequency vibrational modes, and barrier height to internal rotation, which would be helpful in microwave spectroscopy, was missing from these studies. Therefore, the present spectroscopic work was supported by a new series of ab initio calculations at a slightly higher level of theory in order to obtain additional information facilitating the assignment and analysis of the rotational spectrum of CPPB. Received: December 2, 2011 Revised: January 12, 2012 Published: January 12, 2012 1565

dx.doi.org/10.1021/jp211611t | J. Phys. Chem. A 2012, 116, 1565−1570

The Journal of Physical Chemistry A

Article

record the spectrum without a flow system were unsuccessful and only led to the observation of the free cyclopropylphosphine showing once again the easy breaking of the P−B bond in the gas phase at a low partial pressure.3 In the present study the recorded spectra were checked for the cyclopropylphosphine rotational lines,9 but no features that could be attributed to this compound were found. In general, no significant impurities have been observed because all the strongest lines in the spectra were assigned to CPPB. The accuracy of frequency measurement for a strong isolated line is estimated to be 0.03 MHz, which is typical value for the Lille spectrometer. It should be noted that, in the present spectroscopic measurements, the nuclear quadrupole hyperfine structure due to the boron atom was not resolved. The largest hyperfine splittings calculated in the frequency range of the spectrometer (few tens of kHz) are significantly smaller than Doppler-limited line widths (few hundreds of kHz). Therefore, the unresolved hyperfine structure should not influence the observed lineshapes and consequently has no significant effect on the measurement accuracy.

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COMPUTATIONAL METHODS In this study, all the calculations were performed using the Gaussian 03 software package.10 The geometries were fully optimized, and harmonic vibrational frequencies were calculated at the MP2 level, including all electrons in the correlation calculations (MP2(full)) and using the large aug-cc-pVTZ basis set.11 As in ref 1, the geometrical optimizations have resulted in two stable conformations: ap (with H−C−P−B dihedral angle of 180°) and sc (with H−C−P−B dihedral angle of 52°). The transition states existing between stable conformation and the transition state corresponding to eclipsed configuration of BH3 group exhibiting torsional motion (rot-TS) have been characterized using the QST2 procedure implemented in the Gaussian 03 software package. In case of torsion around the C− P bond, two saddle points, each exhibiting only one imaginary frequency, have been found, one with a syn conformation (H− C−P−B dihedral angle of 0°, denoted hereafter syn-TS) and another one with an anticlinal conformation (H−C−P−B dihedral angle of 128°, denoted hereafter ac-TS). The relative energies of each calculated conformation and TS with respect to the ap are listed in Table 1. For all forms, the total electronic

Figure 1. Structures of the ap (a) and sc (b) conformers of cyclopropylphosphine−borane.

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EXPERIMENTAL METHODS

Synthesis. The synthesis of cyclopropylphosphine−borane was performed as previously reported.8 Conventional Absorption Spectroscopy Experiment. The rotational spectrum of CPPB has been recorded using the Lille spectrometer in the frequency range 150−195 GHz. The experimental setup in Lille is a usual absorption type spectrometer built according to a typical scheme, [source of radiation]−[absorption cell]−[detector]. As a source of radiation, we use a two-step frequency multiplication system, based on Agilent E8752D synthesizer (12.5−17 GHz) whose frequency is multiplied by 6 and amplified in the frequency range 75−110 GHz (Spacek Laboratories Inc. active sextupler) at the first stage and additionally multiplied by 2, 3, 5, 6, or 9 (Virginia Diodes Inc. multipliers) at the second. The emitted microwave signal passes through the absorption cell and is detected by an InSb liquid He-cooled bolometer. A 1.2 m Pyrex glass tube with Teflon windows is used as an absorption cell. The experiment has been performed in the so-called “flow mode”. The sample of CPPB was evaporated outside the absorption cell at a temperature close to −20 °C. This temperature was found to be optimal to provide enough vapor pressure in the cell (about 2 Pa) and at the same time to avoid fast decomposition of the sample. Because the cell was kept at room temperature to minimize the observation of spectra of decomposition products, the sample was continuously injected via a side opening at one end of the cell and pumped out via another side opening at the other end. Previous attempts to

Table 1. Relative Energies with Respect to the ap Conformer ΔEapa

a

conformation

ΔEap (kJ/mol)

ap sc ac-TS syn-TS rot-TS

0.00 8.21 (7.28) 15.97 (15.34) 14.41 (13.75) 9.0 (10.0)

ref 1 ΔEap (kJ/mol) 0.00 5.1 11.7 9.6

The ZPE corrected energies are reported in parentheses.

energy has been ZPE corrected with subtraction of the frequency, in the stable conformers, corresponding to the imaginary one in the transition state. As in ref 1, the ap conformer is also the most stable in the present study. The sc conformer is found to be higher in energy by 7.3 kJ/mol, whereas in ref 1 the corresponding energy difference is lower: 5.1 kJ/mol. Compared to the B3LYP 1566



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calculations, the MP2 studies have revealed slightly shorter C− P and P−B bond lengths. In the present work, the C−P bond length is 179.8 pm, whereas in ref 1 it was found to be 181.2 pm. The calculated MP2 P−B bond length is 192.7 pm in comparison with 193.6 pm in ref 1. The shortening of the C−P bond, which is the conformational pathway for the studied compound, could explain rather important changes in relative energies of the transition states. In the present study, the ac-TS and sc-TS were found to lie 15.3 and 13.8 kJ/mol above the energy of the ap. In ref 1, the respective energies of the two TS are significantly lower: 11.7 and 9.6 kJ/mol. The calculated geometrical parameters and harmonic frequencies for the ap and sc conformers of CPPB are given, respectively, in Tables S1 and S2 of the Supporting Information. For comparison, we have also performed ab initio calculations on free cyclopropylphosphine at B3LYP and MP2 levels of theory and using the aug-cc-pVTZ basis set. For the C−P bond, the results are quite similar to those of the title compound, that is, the MP2 value is smaller than the B3LYP value by 1.4 pm (184.0 pm at B3LYP and 182.6 at MP2). One should also note that, compared to the B3LYP values, the calculated MP2 rotational constants of cyclopropylphosphine agree better with the experimental ones.9 The calculated geometrical parameters and rotational constants of cyclopropylphosphine are given in Table S3 of Supporting Information.

programs ASROT and ASFIT by Kisiel14 were used to make predictions and to fit the spectral data. Experimental and theoretical rotational constants of the ap conformer are listed in Table 2. Here we also present the

ASSIGNMENT AND ANALYSIS Both conformations of CPPB are prolate asymmetric rotors, with the asymmetry parameter κ = −0.84 and −0.94 for the ap and sc conformations, respectively. Therefore, the use of a standard Watson A-reduction Hamiltonian in Ir representation12 for treatment of the rotational spectra of the molecule seems to be appropriate. The exception could be made for the description of hindered internal rotation of the borane group where different effective Hamiltonians may be applied.13 According to ab initio calculations, both conformers have relatively high values of a and b projections of dipole moment on inertial axes: μa = 2.9 D and μb = 3.6 D for the ap and μa = 4.5 D and μb = 2.1 D for the sc. Thus, both a- and b-type transitions were expected in the spectra. The recorded experimental spectrum of CPPB revealed to be very dense and to have a relatively low signal-to-noise ratio. The spectra were dominated by a series of strong lines spaced apart approximately by 4.7 GHz. Taking into account the predictions calculated on the basis of the rotational and centrifugal distortion constants from ab initio calculations, these strong lines were identified as unresolved Ka pileups of four rotational transitions having Ka = 0 and 1. Two μa and two μb transitions within the unresolved quartet have the following selection rules: (i) a-type J + 11,J+1 ← J1,J and J + 10,J+1 ← J0,J; (ii) b-type J + 11,J+1 ← J0,J and J + 10,J+1 ← J1,J. The least-squares fit of a series of these unresolved pileups provided correction of B and C rotational constants and new predictions allowing the assignment of the next series of transitions with similar selection rules but for Ka = 1 and 2. The following assignment has been carried out in a straightforward manner when the results of the least-squares fit of transitions with lower values of quantum numbers were used to calculate the predictions of transitions with higher values of quantum numbers. By comparing the experimental values of rotational constants with theoretical ones, we could make a conclusion that the assigned lines belong to the ap conformer of CPPB. The

constants of the same conformer of less abundant 10B isotopic species (relative abundance: 19.9%). Assuming that the substitution of the 11B isotope with 10B does not alter the structure of the molecule, the rotational spectrum of the 10B isotopologue of CPPB was assigned in the following way. First, the shifts between theoretical and experimental rotational constants for the 11B species were calculated. Then, in the ab initio structure, the 11B atom was substituted by 10B, and the set of theoretical rotational constants has been produced for the 10 B species. Finally, the set of rotational constants was corrected using the shifts calculated for the 11B species. The predictions made using these rotational constants and centrifugal distortion parameters of the 11B isotopologue easily allowed assignment of the rotational spectrum of the 10B species. During the assignment of the ground state of the ap conformation, several relatively strong unassigned satellite lines have been identified in the vicinity of Ka = 0, 1 pileups. The relative positions of satellites exhibited smooth dependence on J quantum number, that is, the relative positions could be approximated by a power series containing the terms of J2n type. This is a typical behavior of transitions belonging to excited vibration states. The assignment procedure for the satellite lines was identical to the procedure employed for the ground state, that is, Ka = 0, 1 pileups assigned first and provided accurate predictions for the next series with Ka = 1, 2, and so on. The vibrational assignments were based on relative intensities of the lines. According to ab initio calculations for the ap conformer, the lowest vibrational mode is C−P torsion ν1, which has a frequency that is estimated to be 56 cm−1. At room temperature, the Boltzmann factor of this mode is 0.76. In our spectra, we were able to assign and analyze the rotational transitions belonging to the four lowest excited states ν1 = 1, ..., 4 of the C−P torsional mode. The second and third lowest modes for the ap conformer are B−P−C bending and BH3 torsion, which have fundamentals that are, respectively, 176.4 cm−1 (Boltzmann factor 0.43) and 201.7 cm−1 (Boltzmann

Table 2. Spectroscopic Constants for the Ground States of 11 B and 10B Isotopologues of the ap Conformation of Cylcopropylphosphine−Borane 11

Bv=0

A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) ΦJK (Hz) ΦKJ (Hz) ΦK (Hz) φJ (Hz) Nlines σ (MHz) σwa a

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6717.18124(31) 2668.52264(15) 2338.90141(15) 0.885982(56) 1.50236(23) 2.5660(12) 0.176755(38) −6.8994(14) 0.018896(45) −0.11109(25) 0.1059(13) −0.0005032(90) 651 0.024 0.64

11

B theory

6682.27 2702.46 2357.68 0.903 1.512 2.293 0.186 −8.487

10

Bv=0

6849.27383(66) 2711.80263(42) 2388.44562(31) 0.88258(11) 1.75367(64) 2.2558(27) 0.171679(73) −7.5782(31) 0.02072(14) −0.12004(93) 0.1204(34) [0.0] 305 0.031 0.61

Unitless rms deviation of the fit.



Article

Table 3. Spectroscopic Constants of the Lowest Excited Vibrational States of the 11B Isotopologue of the ap Conformation of Cyclopropylphosphine−Borane

A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) ΦJK (Hz) ΦKJ (Hz) ΦK (Hz) φK (Hz) Nlines σ (MHz) σwa a

ν1 = 1

ν1 = 2

ν1 = 3

ν1 = 4

ν2 = 1

ν1 = 1, ν2 = 1

6715.56568(41) 2670.70590(17) 2341.84841(16) 0.900290(62) 0.50262(27) 3.6167(15) 0.182077(29) −4.6276(16) 0.013075(39) −0.04428(29) 0.0365(16) 0.0 628 0.028 0.66

6718.58846(43) 2669.12711(18) 2342.64950(17) 0.904089(66) 0.24742(31) 3.9488(15) 0.183683(31) −4.0874(17) 0.012121(44) −0.04001(32) 0.0361(16) 0.0 572 0.027 0.57

6725.14641(65) 2664.57277(34) 2341.59445(27) 0.89453(10) 0.32609(57) 3.9819(23) 0.184998(57) −4.2863(27) 0.01116(11) −0.01948(69) 0.0305(26) 0.0 416 0.033 0.69

6733.765(18) 2658.96823(98) 2340.22212(54) 0.90545(23) 0.5119(42) 3.80(23) 0.18568(14) −4.9657(44) 0.0 0.0 0.0 0.0 143 0.036 0.73

6732.0842(18) 2654.3221(11) 2327.29196(46) 0.85960(42) 1.8862(43) 2.7878(38) 0.17223(21) −10.8220(67) −0.1881(49) 0.0 0.0 −2.144(91) 151 0.032 0.64

6730.500(17) 2656.64514(99) 2330.22012(66) 0.88491(20) 0.5457(45) 2.18(22) 0.17771(15) −6.4168(51) 0.0 0.0 0.0 0.0 168 0.038 0.76

Unitless rms deviation of the fit.

Figure 2. Variation of the rotational constants of the ap conformer with the H−C−P−B torsion quantum number.

A and B constants achieve their corresponding minimum and maximum values for ν1 = 1, while the C rotational constant has its maximum value at ν1 = 2. In this case, a quartic−quadratic potential function may be used to model the C−P torsion. For a one-dimensional quartic−quadratic oscillator, the Hamiltonian is expressed as

factor 0.38). For these two modes, we assigned only the rotational transitions in singly excited states ν2 = 1 and ν3 = 1. The rotational parameters obtained as the results of the fit for the excited vibrational states of the ap conformer are listed in Table 3. In addition, a combination vibrational state obtained by excitation of one quantum of ν1 and one quantum of ν2 mode was assigned. Besides relative intensity considerations the correctness of assignment of such a state can be verified using the rotational constants of the ground and singly excited states. Rotational constants of a combination state can be estimated assuming the additivity of the changes in spectroscopic constants when the vibrational quantum numbers are changing: Xi+j = Xi + Xj − X0, where Xi and Xj are A, B, or C rotational constants of singly excited states, which participate in the combination vibration, and X0 is the ground state rotational constant. For example, the calculated value of the A constant is 6730.4686(19) MHz, whereas the value found experimentally is 6730.500(17). Analysis of C−P Torsional Mode. From the dependence of rotational constants on vibrational quantum number, one can determine a potential function governing the corresponding vibrational mode. Being plotted as a function of vibrational quantum number, the behavior of the rotational constants of the C−P torsion exhibits rather regular but strongly anharmonic character (see Figure 2). It can be seen that the

H=

Px2 + Ax4 + Bx 2 2μ

(1)

where x is a generalized vibrational coordinate describing the torsional motion, μ is the reduced mass for this type of vibration, and Px is the vibrational momentum conjugate to x. In reduced form, the Hamiltonian is re-expressed using a transformation of the type x = kz, where k is a function μ and z is a dimensionless coordinate. The Hamiltonian becomes, in terms of z, H = χA(Pz2 + z4 + χBz 2)

(2)

The transformation used in the present study is taken from eq 2 of the paper by Laane.15 1568



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The parameter χB can be derived from torsional dependence of the rotational constants. For a given vibrational state |ν⟩, the rotational constant Bξ, where ξ = a, b, and c can be expressed as ⟨ν|Bξ|ν⟩ = Bξ0 + β2⟨ν|z 2|ν⟩ + β4⟨ν|z4|ν⟩

vibrational state even for transitions with relatively high values of J and Ka quantum number. Furthermore, assigned low-Ka transitions of the first torsional state ν3 = 1 did not exhibit any splitting. Finally, A and E transitions could be resolved in the spectra only for transitions of ν3 = 1 state with Ka ≥ 4. Rather weak signal-to-noise ratio of the observed lines in the first torsional state and possible Coriolis-type interaction with closely lying ν2 = 1 state have limited the number of assigned transitions. Only transitions with Ka ≤ 8 were retained in the final fit. We were able to provide assignment for transitions with higher values of Ka, but these transitions could not be fitted within experimental accuracy. All the assigned rotational transitions in the ν3 = 1 state have been fitted using XIAM code by Hartwig and Dreizler.16 Among 4 parameters describing the internal rotation only the value of the torsional barrier V3 was varied. Due to the limited data on internal rotation splittings the other three parameters (F0, δ and ε) were fixed to values calculated from the ab initio structure of the molecule. Assuming the exact C3v symmetry of the BH3 top, its inertia moment is estimated to be 4.123 uA2. This value is comparable with the value of 4.13 uA2 found for aziridine-borane.7 The inverse value of inertia moment F0 was therefore fixed to 122.576 GHz. The angle between the principal axis z and the internal rotation (angle δ) is fixed to the calculated value of 113.4°. The internal rotation axis lies in the principal xz plane therefore, the angle between the principal axis x and the projection of the internal rotation axis onto xy plane (angle ε) is equal to 0°. The results of the fit are presented in Table 4. It can be seen that the rms deviation of the fit is larger than corresponding

(3)

Having obtained the rotational constants A, B, and C for the ground and several excited vibrational states, we have fit them to eq 3 by a two-step procedure. First, the matrix of the Hamiltonian (eq 2) has been set up and diagonalized in the basis of 100 harmonic oscillator functions. Then the eigenvectors resulting from diagonalization have been used to calculate the expectation values of ⟨ν|zn|ν⟩. The expectation values are used to perform a linear least-squares fit on eq 3. This procedure has been repeated for a wide range of values of χB, and the criterion of the best fit was the smallest standard deviation. The eigenvalues produced by diagonalization of the Hamiltonian (eq 2) correspond to vibrational energy levels and depend only on χA. Therefore, to determine the value of the χA parameter, the differences between the first five energy levels (ν = 0, ..., 4) were fit to experimental values obtained from relative intensities of the rotational lines. As a result, a good agreement between experimental data and theoretical model has been achieved for χA = 12.9 and χB = 3.2. Positive value of χB indicates that the potential has a single minimum corresponding to the value of the H−C−P−B dihedral angle of 180°. According to the calculations the ground state level lies 25 cm−1 above the bottom of the potential well. The shape of the potential function and the position of the lowest energy levels calculated using optimal values of χA and χB parameters are shown in Figure 3.

Table 4. Spectroscopic Constants for the First Excited State ν3 = 1 of BH3 Torsional Mode of the ap Conformation of Cyclopropylphosphine−Borane ν3 = 1 A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) V3 (kJ/mol) F0 (GHz) δ (°) ε (°) Nlines σ (MHz) σwb

6744.038(25) 2659.4570(14) 2332.2297(12) 0.89543(28) 0.6499(66) 4.31(31) 0.18124(26) −3.0331(83) 9.616(15) 122.576a 113.4a 0.0a 160 0.073 0.86

Figure 3. Potential function describing the torsional motion around the C−P bond in the vicinity of the equilibrium position of the ap conformer.

a

Analysis of BH3 Torsional Mode. CPPB belongs to a class of molecules containing one internal rotor of C3v symmetry. The internal rotation of the BH3 top is limited by a 3-fold potential, and each energy level is split in two components of A and E symmetry species of the C3v group. When the potential barrier is sufficiently low A−E splittings are observable in rotational spectra. Under Doppler limited resolution, no splitting due to internal rotation of the BH3 top were observed in the ground

values for other excited vibrational states. This can be explained by the fact that XIAM program does not allow fitting frequency of an unresolved multiplet to a certain mean value calculated using frequencies and relative intensities of each transition in the multiplet. Such a feature is implemented in ASFIT code. Therefore, in XIAM the individual error of each component of multiplet contributes to rms deviation. The calculated height of the barrier to internal rotation V3 = 9.616(15) kJ/mol agrees well with the ZPE corrected value of 10.0 kJ/mol found in quantum chemical calculations. 1569

Value fixed in the fit. bUnitless rms deviation of the fit.



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Failure to Assign Synclinal Conformation. In the present study we were able to assign all the excited vibrational states of the ap conformation up to 280 cm−1 that corresponds to the value of Boltzmann factor of 0.26. The assignment of excited states having higher energies and consequently the Boltzmann factors lower than 0.26 would represent rather heavily task principally due to the density of the rotational spectrum of CPPB. Taking into account the ab initio results on energy difference between the ap and sc conformations (7.3 kJ/ mol) and their respective μa dipole moment components (2.9 D for ap and 4.5 D for sc), the intensities of a-type transitions of the sc at room temperature would be about 0.13 of those of the ap. Therefore, at the current level of experimental signal-tonoise ratio and in view of rather weak relative intensity, the sc would be hardly observable in our spectra.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +33-3-20434490. Fax: +33-3-20337020. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Manuel Goubet and Vadim Ilyushin are gratefully acknowledged for their helpful discussions, respectively, on interpretation of ab initio results and on internal rotation of borane group.

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REFERENCES

(1) Németh, B.; Khater, B.; Guillemin, J.-C.; Veszprémi, T. Inorg. Chem. 2010, 49, 4854. (2) Khater, B.; Guillemin, J.-C.; Benidar, A.; Bégué, D.; Pouchan, C. J. Chem. Phys. 2008, 129, 224308. (3) Németh, B.; Khater, B.; Veszprémi, T.; Guillemin, J.-C. Dalton Trans. 2009, 3526. (4) Bryan, P. S.; Kuczkowsky, R. L. Inorg. Chem. 1972, 11, 553. (5) Durig, J. R.; Brletic, P. A.; Li, Y. S.; Johnston, S. A.; Odom, J. D. J. Chem. Phys. 1981, 75, 1644. (6) Thorne, L. R.; Suenram, R. D.; Lovas, F. J. J. Chem. Phys. 1983, 78, 167. (7) Konovalov, A.; Møllendal, H.; Guillemin, J.-C. J. Phys. Chem. A 2009, 113, 8337. (8) Hurtado, M.; Yánez, M.; Herrero, R.; Guerrero, A.; Dávalos, J. Z.; Abboud, J.-L. M.; Khater, B.; Guillemin, J.-C. Chem.−Eur. J. 2009, 15, 4622. (9) Dinsmore, L. A.; Britt, C. O.; Boggs, J. E. J. Chem. Phys. 1971, 54, 915. (10) Frisch, M. J.; et al. Gaussian 03, Revision D.01; Gaussian, Inc.: Pittsburgh, PA, 2003. (11) Peterson, K. A.; Dunning, T. H. Jr. J. Chem. Phys. 2002, 117, 10548. (12) Watson, J. K. G. Vibrational Spectra and Structure; Elsevier: Amsterdam, 1977; Vol. 6, p 1. (13) Kleiner, I. J. Mol. Spectrosc. 2010, 260, 1. (14) Kisiel, Z. Spectroscopy from Space; Kluwer Academic Publishers: Dordrecht, 2001; p 91; see also the webpage at http://www.ifpan.edu. pl/∼kisiel/prospe.htm. (15) Laane, J. Appl. Spectrosc. 1970, 24, 73. (16) Hartwig, H.; Dreizler, H. Z. Naturforsch. 1996, 51, 923. (17) Cole, G. C.; Møllendal, H.; Guillemin, J.-C. J. Phys. Chem. A 2005, 109, 7134. (18) Cole, G. C.; Mollendal, H.; Guillemin, J.-C. J. Phys. Chem. A 2006, 110, 2134.

CONCLUSIONS In the present paper we report the high resolution study of the rotational spectrum of cyclopropylphosphine−borane. Quantum chemical calculations suggest that the studied compound has two conformations. The ap conformation is predicted to be the most stable by quantum chemical calculations and confirmed by spectroscopic observations being the only conformer assigned in the spectra. The ab initio calculations at MP2 level and using aug-cc-pVTZ basis set predict the sc conformer to be less stable by 7.3 kJ/mol. Therefore, the sc would hardly be observable in the recorded spectra taking into account the Boltzmann factor, the density of the spectrum, and the signal-to-noise ratio. The analysis of the lowest excited vibrational states enabled determination of potential function governing the C−P torsional motion in the vicinity of equilibrium position of the ap conformer. It was found that the potential exhibits anharmonic character, having rather broad and flattened well. The internal rotation of BH3 top was also studied on the basis of limited number of A−E splitting observed for the rotational lines of the first excited state corresponding to this torsion. The value of the barrier to internal rotation found to be 9.616(15) kJ/mol is in agreement with the value of 10.0 kJ/mol obtained from ab initio calculations. It can be also compared with barrier heights for other compounds containing borane group: 8.565(38) kJ/mol6 for H3N−BH3 and 12.1(3) kJ/mol7 for C2H5N−BH3. In the cyclopropyl derivatives bearing an acidic hydrogen on the γ-position of the cyclopropyl ring, cyclopropylmethylphosphine17 cyclopropyl-methaneselenol,18 a close approach between a hydrogen atom belonging to the hetero group and the edge of the cyclopropyl ring, allows an energetically preferred conformer. At the opposite, the presence of a phosphine−borane group, for which the hydrogens on the boron atom are not acidic, does not promote the presence of such a conformer.

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Article

ASSOCIATED CONTENT

* Supporting Information S

Calculated molecular structure and harmonic vibrational frequencies, rotational line assignments, measured frequencies, experimental uncertainties, and deviations from the final fits for studied ground and excited vibrational states of 10B and 11B isotopologues of cyclopropylphosphine−borane, as well as calculated molecular structures of cyclopropylphosphine. This material is available free of charge via the Internet at http:// pubs.acs.org. 1570


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