Article pubs.acs.org/JPCA
Microwave Spectrum, Conformational Properties, and Dipole Moment of Cyclopropylmethyl Isocyanide (C3H5CH2NC) Svein Samdal,† Harald Møllendal,*,† and Jean-Claude Guillemin‡ †
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway ‡ Institut des Sciences Chimiques de Rennes, École 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 cyclopropylmethyl isocyanide, C3H5CH2NC, has been investigated in the 25−75 GHz spectral range. The spectra of two conformers were assigned. The H−C−C−N chain of atoms is antiperiplanar in the conformer denoted ap and synclinal in the sc rotamer. The sc conformer tends to be slightly more stable than the ap form. The internal energy difference was determined to be Eap − Esc = 0.2(7) kJ/mol from relative intensity measurements. The spectra of the ground vibrational state and six vibrationally excited states belonging to two different normal vibrations were assigned for sc. The frequencies of these two modes were determined by relative intensity measurements. The dipole moment of this conformer was determined to be μa = 12.16(6), μb = 5.91(4), μc = 0 (preset), and μtot = 13.52(6) × 10−30 C m [4.05 (2) debye]. The spectra of the ground and of two vibrationally excited states belonging to the torsion and lowest bending vibration were assigned for ap. The microwave work was supported by quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. Most, but not all, of the theoretical predictions are in good agreement with experiment.
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INTRODUCTION Our two laboratories have recently become interested in organic isocyanide compounds because this functional group has an interesting and unique chemistry that has been comparatively little investigated.1−3 Our isocyanide studies have concentrated on the syntheses4 of several members of this class of compounds, which have subsequently been investigated by UV photon electron spectroscopy,4 microwave (MW) spectroscopy,5−8 and high-level quantum chemical calculations.4−8 So far, we have reported microwave spectra of allenyl isocyanide (H2CCCHNC),5 2-fluoroethyl isocyanide (FCH2CH2NC),6 2-chloroethyl isocyanide (ClCH2CH2NC),7 and E- and Z-1-propenylisocyanide (CH3CHCHNC).8 Our MW studies of H2CCCHNC5 and E- and Z-CH3CHCHNC8 were undertaken because of their potential astrochemical interest, while conformational properties were in focus in our investigations of 2-fluoroethyl6 and 2-chloroethyl7 isocyanide. In this work, our isocyanide studies are extended to the first MW investigation of the conformational and structural properties of cyclopropylmethyl isocyanide (C3H5CH2NC). A model of two typical conformers of this compound with atom numbering is shown in Figure 1. The orientation of the H6−C2−C9−N12 chain of atoms can conveniently be used to describe these rotamers. In the conformer denoted ap, the said link of atoms are antiperiplanar and form a dihedral angle of 180°. This conformer has a symmetry plane bisecting the cyclopropyl © XXXX American Chemical Society
ring. In the second rotamer denoted sc, the H6−C2−C9−N12 chain has a synclinal orientation and has a dihedral angle of about 60°. Obsolete nomenclature, “cis” and “gauche” for ap and sc, respectively, is often encountered for other methyl substituted cyclopropane conformers. Experimental conformational and structural studies using MW, infrared, and Raman spectroscopy have been reported for many substituted methylcyclopropane derivatives (C3H5CH2X) including X = F,9−12 Cl,10,13−16 Br,10,14,16,17 I,10,18 OH,19 SH,20 SeH,21 NH2,22 PH2,23 CH3,24 SiH3,25−28 SiF3,25,29,30 CN,31,32 and CC−H.33,34 The last two compounds, C3H5CH2CN and C3H5CH2CCH, are of special relevance for the title compound because these three molecules are isoelectronic and have one triple bonded substituent each attached to the methyl group. It has generally been found in these comprehensive studies of substituted methylcyclopropane derivatives (C3H5CH2X) that the sc form predominates and that this form becomes increasingly more stable as the size of the substituent X increases. Steric effects are therefore important for the conformational preferences of these molecules. The gauche effect35 should also favor the sc orientation, especially when X is electronegative. Intramolecular hydrogen Received: April 5, 2013 Revised: May 16, 2013
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Figure 1. Models of the H6−C2−C9−N12 antiperiplanar (ap) and synclinal (sc) conformers of cyclopropylmethyl isocyanide. sc was found to be 0.2(7) kJ/mol more stable than ap from relative intensity measurements.
bonding with the pseudo-π electrons36 of the cyclopropyl ring will stabilize the sc forms, as discussed in the cases of X = OH,19 SH,20 SeH,21 NH2,22 and PH2,23 as well as in a review.37 However, there is one important exception to this preference of the sc conformer, namely, C3H5CH2CCH, where ap is 0.77(36) kJ/mol more stable than sc.33 However, its isoelectronic cyano derivative, C3H5CH2CN, follows the general trend and only the MW spectrum of sc was assigned in this case, and this rotamer is present in the gas phase in at least 85% concentration at room temperature according to an infrared study.31 The two examples, C3H5CH2CN and C3H5CH2CCH, demonstrate that the conformational preferences of these isoelectronic compounds are quite sensitive to the nature of the triple bond. The question whether the corresponding isocyanide derivative C3H5CH2NC behaves in a way similar to that of its isoelectronic cyano relative, C3H5CH2CN, or similar to its acetylene congener, C3H5CH2CCH, was another motivation to undertake the present first MW study of cyclopropylmethyl isocyanide. Our choice of experimental method is MW spectroscopy, due to its superior accuracy and resolution, which makes this method ideal for conformational and structural studies. The MW investigation has been augmented with high-level quantum chemical calculations, which were conducted with the purpose of obtaining information for use in assigning the MW spectrum and investigation properties of the potential-energy hypersurface.
the microwave spectrometer of the University of Oslo. Details of the construction and operation of this device have been given elsewhere.38−40 This spectrometer has a resolution of about 0.5 MHz and measures the frequency of isolated transitions with an estimated accuracy of ∼0.10 MHz. The spectrum was investigated in the whole 25−75 GHz frequency interval. Selected measurements were also performed in other frequency regions. Radio-frequency microwave double-resonance experiments (RFMWDR), similar to those undertaken by Wodarczyk and Wilson,41 were also conducted to unambiguously assign particular transitions, using the equipment described elsewhere.38
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RESULTS AND DISCUSSION
Quantum Chemical Methods. The present ab initio coupled clusters singlet and double substitutions42−45 (CCSD) and density functional theory (DFT) calculations were performed employing the Gaussian 0946 program package running on the Abel cluster in Oslo. Becke’s three-parameter hybrid functional employing the Lee, Yang, and Parr exchange-correlation functional (B3LYP)47 was employed in the DFT calculations. Peterson and Dunning’s48 correlation-consistent cc-pVTZ basis set, which is of triple-ζ quality, were used in all the calculations. Quantum Chemical Calculations. B3LYP/cc-pVTZ calculations of the optimized structures and dipole moments of ap and sc were first performed. All structural parameters were varied freely in these calculations with no symmetry restrictions. The vibrational frequencies, Watson’s A-reduction quartic and sextic centrifugal distortion constants,49 and the vibration−rotation interaction constants50 (the αs) were then calculated using the principal inertial axes coordinates of the optimized structures, which were kept fixed in the calculations, as pointed out by McKean et al.51 The results of these calculations are found in Tables 1S and 2S of the Supporting Information. Centrifugal distortion constants are repeated in the last columns of Tables 2 and 4 for comparison with their experimental counterparts. It is seen from Tables 1S and 2S, Supporting Information, that no imaginary harmonic normal vibrations were obtained for any of the two forms, which is an indication that ap and sc are indeed minima on the potential-energy hypersurface. ap was found to have a symmetry plane consisting of the H6−C2−C9−N12−C13 link of atoms. The important H6−C2−C9−N12 dihedral angle was found to be 62.4° in sc. The B3LYP energy difference corrected for the zero-point vibrational effect is 2.25 kJ/mol, with sc as the more stable rotamer, whereas the electronic energy difference is 1.86 kJ/mol.
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EXPERIMENTAL SECTION Synthesis. Cyclopropylmethyl isocyanide has been synthesized starting from the corresponding formamide as described previously4 (Scheme 1). Scheme 1
Spectroscopic Experiments. Cyclopropylmethyl isocyanide is a colorless liquid at room temperature with a vapor pressure of roughly 90 Pa. The MW spectrum was recorded at room temperature or with the cell cooled to about −30 °C using small portions of dry ice to cool the waveguide. The pressure was roughly 7 Pa during the measurements. The MW spectrum was studied with Stark-modulation spectroscopy using B
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Table 1. CCSD/pVTZ Structures, Dipole Moments, and 14N Nuclear Quadrupole Coupling Constants of the ap and sc Conformers of C3H5CH2NC conformer C1−C2 C1−C3 C1−H4 C1−H5 C2−C3 C2−H6 C2−C9 C3−H7 C3−H8 C9−H10 C9−H11 C9−N12 N12−C13 C2−C1−H4 C2−C1−H5 C3−C1−H4 C3−C1−H5 H4−C1−H5 C1−C2−H6 C1−C2−C9 C3−C2−H6 C3−C2−C9 H6−C2−C9 C1−C3−H7 C1−C3−H8 C2−C3−H7 C2−C3−H8 H7−C3−H8 C2−C9−H10 C2−C9−H11 C2−C9−N12 H10−C9−H11 H10−C9−N12 a
ap Bond Distance (pm) 150.2 150.9 108.0 108.0 150.2 108.2 151.3 108.0 108.0 109.0 109.0 142.9 116.9 Angle (deg) 117.8 117.7 118.4 116.9 115.2 116.5 121.4 116.5 121.4 111.9 116.9 118.4 117.7 117.8 115.2 110.4 110.4 112.3 108.1 107.7
conformer
sc
ap
sc
Angle (deg) H11−C9−N12 107.7 107.9 C9−N12−C13 178.8 178.7 Dihedral Angle (deg) H4−C1−C2−H6 1.5 0.7 H4−C1−C2−C9 −140.9 −144.0 H5−C1−C2−H6 146.5 144.7 H5−C1−C2−C9 4.1 0.0 H4−C1−C3−H7 144.7 144.7 H4−C1−C3−H8 0.0 −0.1 H5−C1−C3−H7 0.0 −0.4 H5−C1−C3−H8 −144.7 −145.2 H6−C2−C3−H7 −146.5 −145.3 H6−C2−C3−H8 −1.5 −1.1 C9−C2−C3−H7 −4.1 −1.4 C9−C2−C3−H8 140.9 142.8 C1−C2−C9−H10 84.2 −157.2 C1−C2−C9−H11 −156.3 −37.6 C1−C2−C9−N12 −36.1 82.3 C3−C2−C9−H10 156.3 −87.6 C3−C2−C9−H11 −84.2 32.0 C3−C2−C9−N12 36.1 151.9 H6−C2−C9−H10 −59.7 57.2 H6−C2−C9−H11 59.7 176.9 H6−C2−C9−N12 180.0 −63.2 Electronic Energy Differencea (kJ/mol) 0.16 0.0 Dipole Momentb (10−30 C m) μa 8.37 11.4 μb 8.47 5.42 μc 0.0c 0.86 μtot 11.9 12.6 Principal-Axis 14N Quadrupole Coupling Constants (MHz) χaa 0.0596 0.1456 χbb 0.0427 −0.0353
150.1 150.8 107.9 108.1 150.4 108.1 150.6 108.1 107.9 109.0 109.0 143.2 116.9 117.9 117.4 118.2 117.8 114.8 117.0 118.9 117.2 118.2 114.7 117.7 113.3 117.7 117.9 114.7 111.2 110.1 111.6 108.0 107.9
The CCSD/cc-pVTZ electronic energy of the sc conformer: −653542.38 kJ/mol. The energy of the ap form is 0.16 kJ/mol higher than the energy of the sc rotamer. b1 debye = 3.33564 × 10−30 C m. cFor symmetry reasons.
inertial axis components of nuclear quadrupole coupling tensor of the 14N nucleus are included in Table 1. The rotational constants obtained from the CCSD structures are shown together with their experimental equivalents in the last columns of Tables 2 and 4. ap has a symmetry plane (Cs symmetry) consisting of the H6−C2−C9−N12−C13 plane of atoms. This plane bisects the cyclopropyl ring. The planar moment, defined by Pcc = (Ic − Ia − Ib)/2, where Ia, Ib, and Ic, are the principal moments of inertia, varies when a vibrational mode is excited and can be used to identify the nature of the vibration.52,53 The theoretical Pcc is useful for comparison with experiment and is therefore shown in the last column of Table 4. Some of the CCSD results warrant further comments. The CCSD electronic energy difference is only 0.16 kJ/mol with sc as the more stable conformer, compared to the B3LYP value (1.86 kJ/mol) given above. The C−C bond lengths of the cyclopropyl ring varies between 150.1 to 150.9 pm (Table 1), compared to the equilibrium bond length in cyclopropane, which is 151.0077(77) pm.54 The variation of the bond lengths of the cyclopropyl ring is interesting. The C1−C3 bond length, which is opposite to the methyl
The B3LYP potential function for rotation about the C2−C9 bond was also calculated using the scan option of Gaussian09. The H6−C2−C9−N12 dihedral angle was stepped in 10° intervals in these calculations. The function, which is drawn in Figure 2, has maxima at 0 and 123.2°. The energies at these transitions states are 14.52 (0°) and 13.01 kJ/mol (123.2°) higher than the electronic energy of sc. The transitions state structures and energies were obtained employing the Gaussian09 transition-state option. Finally, comprehensive CCSD/cc-pVTZ calculations of optimized structures, dipole moments, electronic energies, and nuclear quadrupole coupling constants of the 14N nucleus of the ap and sc were performed using the B3LYP structures in Tables 1S and 2S, Supporting Information, as starting points. ap was assumed to have a symmetry plane in these calculations to save computational time. Unfortunately, it is not possible to calculate vibrational frequencies and centrifugal distortion constants at the CCSD with our present computational resources. The resulting CCSD structures are listed in Table 1, while the CCSD principal inertial coordinates of the atoms are listed in Table 3S (ap) and 4S (sc) of the Supporting Information. The electronic energy difference, the dipole moments, and the principal C
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Table 2. Spectroscopic Constantsa of the sc Conformer of C3H5CH2NC A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δk (kHz) ΦJ (Hz) ΦKJ (Hz) ϕJd (Hz) rmse Nf
ground state
first ex. tors.
second ex. tors.
third ex. tors.
fourth ex. tors.
lowest bend.
comb. state
theoryb
10841.5285(84) 2134.7048(12) 1935.7963(12) 0.9055(24) −9.350(11) 59.21(37) 0.00090(15) 3.4329(76) −0.0196(12) −2.15(19) 0.00090(15) 1.508 364
10772.923(12) 2137.9832(26) 1938.5463(25) 0.9519(96) −9.197(14) 54.19(43) 0.19442(13) 3.5489(92) 0.024(11) −1.90(24) 0.000974(31) 1.564 283
10707.747(14) 2141.1014(22) 1941.2142(21) 0.9741(41) −9.020(14) 47.48(60) 0.19523(18) 3.616(15) 0.02387c −1.897c 0.000698(38) 1.628 159
10646.417(29) 2144.0599(36) 1943.7890(36) 1.023(14) −9.040(48) 47.2(11) 0.19800(72) 3.965(32) 0.096(17) −1.7(10) 0.00124(29) 1.665 148
10589.865(40) 2146.8078(29) 1946.2808(28) 1.0432(58) −8.936(28) 50.7(12) 0.15081(43) 9.050(60) 0.09644c −1.65c 0.0012388c 1.844 94
10913.539(20) 2138.5945(27) 1938.2801(26) 0.9370(53) −9.450(21) 65.57(97) 0.19509(25) 3.813(33) −0.019554c −2.148c 0.00090987c 1.977 170
10839.167(42) 2141.9373(24) 1941.1274(24) 0.9444(45) −9.770(77) 71.9(17) 0.19666(52) 3.905(41) −0.019554c −5.7(16) 0.00141(17) 1.844 155
10939.3 2130.7 1935.0 0.791 −8.16 58.8 0.159 3.05
a
A-reduction Ir-representation.49 Uncertainties represent one standard deviation. The spectra are found in the Supporting Information in Table 5S (ground state), Tables S6−S9 (successive excited states of the torsion), Table 10S (lowest bending vibration), and 11S (combination state of the first excited state of the torsion and first excited state of the lowest bending vibration). bThe theoretical rotational constants have been calculated from the CCSD structure in Table 1, whereas the centrifugal distortion constants were obtained in the B3LYP calculations. cFixed. dFurther sextic constants preset at zero; see text. eRoot-mean-square deviation defined as rms2 = Σ[(νobs − νcalcd)/u]2/(N − P), where νobs and νcalcd are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P is the number spectroscopic constants used in the fit. fNumber of transitions used in the fit.
canonical 60°. This CCSD angle is similar to 62.4° found in the B3LYP calculations (Table 2S, Supporting Information). Comparatively small values were calculated for the 14N quadrupole coupling constants (Table 1). The small quadrupole coupling constants obtained in these calculations are typical for isocyanides. The quadrupole coupling constant of the 14N nucleus of CH3NC is, for example, only 0.4894(4) MHz.57 Microwave Spectrum and Assignment of the Spectrum of sc. The MW spectrum was found to be of moderate intensity and dense with absorption lines occurring every few MHz in the whole investigated spectral range. A typical example is shown in Figure 3. The high spectral density is understandable
Figure 2. B3LYP/cc-pVTZ barrier to internal rotation about the C2−C9 bond. This curve has minima at 62.4° (sc) and 180° (ap). The energy difference between these two forms including zero-point vibrational effects is 1.86 kJ/mol favoring sc, compared to 0.2(7) kJ/mol found experimentally. The two maxima at 0 and 123.2° have energies that are 14.52 and 13.01 kJ/mol higher than the B3LYP energy of sc.
ioscyanide substituent, is slightly longer by 0.4−0.7 pm than the adjacent C1−C2 and C2−C3 bond lengths. This asymmetry is typical for substituted cyclopropanes.55 The NC bond length is 116.9 pm in both conformers, almost the same as the equilibrium NC bond length in H−NC, which is 116.83506(16) pm.56 Comparatively small variations (∼1°) are seen for the majority of the bond angles in the two forms. There is a notable exception. The C1−C2−C9 and the C3−C2−C9 angles are both 121.4° for symmetry reasons in ap. In sc, these angles are 118.9 and 118.2°, respectively. Similar differences for these angles were predicted in the B3LYP calculations (Table 1S and 2S, Supporting Information). Interestingly, the H6−C2−C9−N12 dihedral angle, which in a sense describes the conformational properties of this compound, is 63.2° in sc, three degrees larger than the
Figure 3. Portion of the MW spectrum taken at a field strength of about 110 V/cm demonstrating the complexity of the spectrum. This spectral region is dominated by absorption lines mainly associated with the J = 17 ← 16 a-type transitions. Values of the K−1 pseudo quantum number is listed above several peaks belonging to the ground vibrational state. Lines with K−1 quantum numbers between 8 and 12 coalesce at a frequency of about 69250 MHz. Pairs with K−1 quantum numbers 7, 6, and 5 coalesce, whereas the K−1 = 4 pair is split. Most of the remaining unlabeled transitions belong to vibrationally excited states. D
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Table 3. Stark Coefficientsa and Dipole Momenta of the sc Conformer of C3H5CH2NC
since both ap and sc have comparatively large dipole moment components along the a- and the b-inertial axes (Table 1) according to the CCSD calculations above, as well as the fact that spectra of vibrationally excited states of both rotamers might contribute significantly to the spectrum because there are five fundamental vibrational normal modes of ap and six of sc below 500 cm−1 (Tables 1S and 2S of the Supporting Information). The quantum chemical calculations predict that sc is the slightly more stable rotamer. This form has a statistical weight that is twice that of ap because there are two mirror-image forms of this conformer. Searches were therefore first made for aR-lines of sc using the RFMWDR technique.41 The approximate frequencies of these transitions were predicted using the CCSD rotational constants and the B3LYP quartic centrifugal distortion constants shown in the last column of Table 2. The RFMWDR experiments met with success and secured the first unambiguous assignments. An example of a RFMWDR spectrum is shown in Figure 4. The transitions assigned in this manner were weighted
Stark coefficients ΔE−2/(10−6 MHz V−2)cm2 transitions 41,4 ← 31,3
41,3 ← 31,2 51,4 ← 41,3 61,6 ← 51,5 60,6 ← 50,5 71,7 ← 61,6 μa = 12.16(6) a
|M| 0 1 2 1 1 0 1 1 3 1 Dipole Momentb μb = 5.91(4)
obs
calcd
−3.52(6)a 14.3(3) 68.6(8) −19.8(4) −8.74(10) −3.80 (6) −2.72(4) −2.49(5) 4.58(10) −18.4(3) (10−30 C m) μc = 0c
−3.54 14.5 68.7 −19.8 −8.68 −3.94 −2.70 −2.35 4.52 −18.5 μtot = 13.52(6) b
Uncertainties represent one standard deviation. Conversion factor: 1 debye = 3.33564 × 10−30 C m. cPreset at this value; see text.
coupling constants calculated for this nucleus (Table 1). Ultimately, nearly 400 lines were assigned; 364 of them, shown in Table 5S of the Supporting Information, were used to determine the spectroscopic constants displayed in Table 2. The maximum value of J is 64. Transitions involving even higher values of J were searched for, but these lines were too weak to be assigned unambiguously. Their weakness is presumably due to an unfavorable Boltzmann factor. It is seen from Table 2 (second column) that the dimensionless root-mean-square deviation is 1.51 and that accurate values have been obtained for the rotational constants and the quartic centrifugal distortion constants. Three of the sextic centrifugal distortion constants, ΦJ, ΦKJ, and ϕJ, were also determined. Attempts to determine significant values for additional sextic centrifugal distortion constants failed, and these constants were therefore preset at zero in the least-squares fit. The experimental rotational constants (first column; Table 2) and the CCSD rotational constants (last column of the same table) agree to within 1% or better. A difference this small is to be expected because the experimental rotational constants referred to an ef fective structure, whereas the CCSD rotational constants have been obtained from the approximate equilibrium structure shown in Table 1. The fact that the CCSD and experimental rotational constants are so similar is reassuring and is an indication that the CCSD structure is indeed very accurate, as expected for this compound composed of elements of the first and second row of the periodic table. The experimental quartic centrifugal distortion constants deviate comparatively much more from their B3LYP counterparts than the rotational constants do. The smallest relative deviation is found for ΔK (0.7% deviation), while the largest deviation is seen for δJ (17%). The experimental sextic constants are not sufficiently accurate to warrant a comparison with their theoretical counterparts. Vibrationally Excited States of sc. The ground-state transitions were accompanied by series of weaker lines with very similar Stark effects and RFMWDR patterns. These lines were assumed to belong to the spectra of vibrationally excited states. The six lowest normal modes of the sc conformer have anharmonic frequencies of 87, 148, 273, 309, 352, and 477 cm−1, according to the B3LYP calculations (Table 2S of the Supporting Information). It is seen from Table 2 that six excited states were assigned. Four of these states belong to successively excited states
Figure 4. RFMWDR spectrum of the 154,12 ← 144,11 and 154,11 ← 144,10 transitions of the ground and vibrationally excited states of sc using a RF of 10.15 MHz. These two transitions are split by about 10 MHz. The ground-state transitions are found at 61179.76 and 61190.14 MHz, respectively. Most of the vibrationally excited states have higher frequencies. Brackets are used to indicate 154,12 ← 144,11 and 154,11 ← 144,10 pairs of the ground and several vibrationally excited states.
according to their estimated uncertainties and least-squares fitted to Watson’s Hamiltonian using the A-reduction Ir-representation,49 as implemented in Sørensen’s program Rotfit58 to obtain a set of preliminary spectroscopic constants that were employed to predict the approximate frequencies of additional a-type R-branch lines, which were subsequently found with ease. Their Stark effects and fit to Watson’s Hamiltonian confirmed their assignment. The preliminary spectroscopic constants thus obtained were now used to predict the frequencies of bQ-transitions. These transitions were also found close to their predicted frequencies. The assignments were then gradually extended to include additional a- and b-type lines of higher and higher values of the principal quantum number J and the pseudoquantum number K−1. Searches were also made for c-type lines, but none were found, presumably because of the small μc dipole moment (see below, Table 3). No lines obviously split by nuclear quadrupole interactions of the 14N nucleus were seen, which is in accord with the comparatively small quadrupole E
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Assignment of the Ground State Spectrum of ap. This conformer has a significant μa and the RFMWDR technique was therefore employed to obtain the first unambiguous transitions of the spectrum of ap, which was found to be significantly less intense than that of sc. The assignments were gradually extended to include additional a- and b-type transitions in a manner very similar to that of sc, as described above. Ultimately, roughly 550 transitions were assigned; 526 of which (Table 11S, Supporting Information) were used to determine the spectroscopic constants shown in Table 4. The maximum values of J and K−1 are 70 and 22, respectively. Searches were made for transitions involving even higher values of J and K−1, but they were too weak to be unambiguously assigned, presumably due to an unfavorable Boltzmann factor. All five quartic and six of the seven sextic centrifugal distortion constants were determined. The exception is ϕJ, which was preset at zero in the least-squares fit. It is seen from Table 4 that the experimental and CCSD rotational constants deviate by less than 1%, which is an indication that the CCSD structure is very close to the equilibrium structure, as anticipated. Moreover, the good agreement between the experimental (−21.684855(25) × 10−20 u m2) and theoretical (−21.59 × 10−20 u m2) planar moments (Pcc) is additional evidence of the quality of the CCSD structure. Interestingly, the values of Pcc found for the corresponding ap conformer of the isoelectronic compound C3H5CH2CCH calculated from the reported rotational constants33 is −21.67 × 10−20 u m2, very similar to the corresponding value of the title compound (−21.684855(25) × 10−20 u m2; Table 4). The best agreement between the B3LYP quartic centrifugal distortion constants and their experimental counterparts (same table) is seen for ΔK (2%). The worst case is δJ (13%). The experimental and theoretical sextic constants deviate so much that a comparison is meaningless. Obviously, more sophisticated quantum chemistry approaches are needed to improve agreement between theory and experiment, but this was not possible with the computational resources available for us. The dipole moment could not be determined because relevant transitions had insufficient intensities to allow a quantitative determination of their Stark effects. Vibrationally Excited States of ap. This rotamer has five anharmonic vibrational fundamentals at 85, 135, 273, 296, and 334 cm−1, according to the B3LYP calculations (Table 1S, Supporting Information). The spectra of two vibrationally excited states were assigned. Only aR-lines were found in these cases. The spectra consisting of 81 and 73 transitions, respectively, are shown in Tables 13S and 14S of the Supporting Information, while the spectroscopic constants are listed in Table 4. Only three centrifugal distortion constants, ΔJ, ΔJK, and δJ, were varied in the least-squares fit of the spectra of these two excited states, with the remaining centrifugal distortion constants kept fixed at the ground-state values. The result is shown in Table 4, columns 3 and 4. Relative intensity measurements yielded 77(30) cm−1 for the first of these modes, which is presumed to be the first excited state of the torsion about the C2−C9 bond. This frequency should be compared with the B3LYP anharmonic frequency of 85 cm−1 (Table 1S, Supporting Information). The vibration− rotation constants calculated for the torsion from the entries in Table 3 are αA = 20.37(64), αB = 16.179(28), and αC = 4.104(32) MHz, which compare rather poorly with the B3LYP values of −11.58, 12.76, and 4.26 MHz, respectively (Table 1S, Supporting Information). The largest discrepancy is seen for
of the torsion about the C2−C9 bond (columns 3−6), one to the lowest bending vibration (column 7), and one was assigned as a combination mode consisting of the first excited state of the torsion and the first excited state of the bending vibration (column 8). The assignments were performed in a manner analogous to that of the ground state. The spectrum of the most intense excited state (Table 6S, Supporting Information) has an intensity of roughly 65% of that of the ground vibrational state. Relative intensity measurements performed largely as described by Esbitt and Wilson59 yielded 73(20) cm−1 for this mode, which is assumed to be the first excited state of the torsion about the C2−C9 bond, whose B3LYP anharmonic frequency is 87 cm−1. It is possible to compare the experimental and theoretical vibration−rotation constants defined by αex = X0 − Xex,50 where X0 is the rotational constants of the ground state and Xex are the rotational constants of the excited state under consideration. The experimental vibration−rotation constants derived from the entries in columns 2 and 3 of Table 2 are αA = 68.6055(15), αB = −3.2784(29), and αC = −2.7500(28) MHz, compared to the B3LYP values of 71.68, −2.15, and −2.03 MHz, respectively (Table 2S, Supporting Information). The agreement is satisfactory given the approximate nature of the B3LYP calculations. Spectra of four successively excited torsional states were assigned (Tables 6S−10S, Supporting Information); the spectroscopic constants are shown in Table 2. It is seen that the rotational constants of these states change in a regular fashion upon excitation, which is typical for an essentially harmonic vibration.52 An exited-state spectrum (Table 11S, Supporting Information) assumed to belong to the first excited state of the lowest bending vibration was also assigned and its frequency was determined to be 138(30) cm−1 by relative intensity measurements, compared to the theoretical anharmonic frequency, which is 148 cm−1 (Table 2S, Supporting Information). The vibration−rotation constants derived for this mode from columns 2 and 7 of Table 2 are αA = −72.011(22), αB = −3.8897(30), and αC = −2.4838(29) MHz, in reasonable agreement with the B3LYP values of −82.61, −3.21, and −1.93 MHz, respectively (Table 2S, Supporting Information). Finally, the spectrum of a combination mode of the torsion and bending vibrations were assigned. The vibration−rotation constants derived from columns 2 and 8 of Table 2 are αA = 2.362(43), αB = −7.2325(27), and αC = −5.3311(27) MHz, close to −3.41, −7.17, and −5.23 MHz, respectively, obtained by simple addition of the αs of the torsion and bending vibrations given above. Dipole Moment of sc. The dipole moment was determined by least-squares fitting the second-order Stark coefficient shown in Table 3. The weight of each Stark coefficient was taken to be the inverse square of its standard deviation, which is also shown in the same table. The theoretical values of the second-order Stark coefficients were calculated as described by Golden and Wilson60 using program MB04.61 μc is small, and it was not possible to obtain it experimentally. This dipole moment component was therefore preset at zero in the least-squares fit. The experimental dipole moment components are μa = 12.16(6), μb = 5.91(4), μc =0.0 (preset), and μtot = 13.52(6) × 10−30 C m [4.05 (2) debye], where the uncertainties represent one standard deviation. The experimental values are in quite good agreement with the calculated dipole moments at the CCSD/cc- pVTZ level of theory (12.1, 5.67, 0.96, and 13.4 × 10−30 C m, respectively; Table 1). F
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Table 4. Spectroscopic Constantsa of the ap Conformer of C3H5CH2NC A (MHz) B (MHz) C (MHz) Pccc (10−20 u m2) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δk (kHz) ΦJ (Hz) ΦJK (Hz) ΦKJ (Hz) ΦK (Hz) ϕJ (Hz) ϕJK (Hz) ϕK (Hz) rmse Nf
ground state
first ex. tors.
lowest bend.
theoryb
7122.0063(35) 2827.5981(11) 2449.4752(11) −21.684855(25) 1.7491(27) −4.8509(37) 11.364(31) 0.42012(16) −2.5210(54) −0.0070(14) −0.0741(63) 0.130(42) −0.600(71) 0.0c 0.0811(28) −1.04(13) 1.737 526
7101.69(64) 2811.422(28) 2445.371(32) −22.125(5) 1.801(12) −4.356(45) 11.364d 0.101(34) −2.5210d −0.0070d −0.0741d 0.130d −0.600d 0.0d 0.0811d −1.04d 1.676 81
7160.62(42) 2829.264(21) 2446.291(18) −21.306(4) 1.767(21) −5.409(58) 11.364d 0.445(39) −2.5210d −0.0070d −0.0741d 0.130d −0.600d 0.0d 0.0811d −1.04d 2.159 77
7180.2 2816.1 2445.4 −21.59 1.58 −4.50 11.6 0.366 −2.76 0.00118 −0.00199 −0.0791 0.127 0.000610 0.0280 0.527
a A-reduction Ir-representation.49 Uncertainties represent one standard deviation. The spectra are listed in Tables 12S−14S in the Supporting Information. bThe rotational constants were calculated from the CCSD structure in Table 1. cDefined by Pcc = (Ic − Ia − Ib)/2, where Ia, Ib, and Ic, are the principal moments of inertia. Conversion factor: 505379.05 × 10−20 MHz u m2. dFixed. eRoot-mean-square deviation defined as rms2 = Σ[(νobs − νcalcd)/u]2/(N − P), where νobs and νcalcd are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P is the number spectroscopic constants used in the fit. fNumber of transitions used in the fit.
αA. Calculation of the vibration−rotation constants requires the third derivative of the energy with respect to the structure and deviations such as those seen for αA are to be expected due to the approximation involved in B3LYP/cc-pVTZ calculations. The second excited state has a frequency of 127(30) cm−1 by relative intensity measurements, compared to 135 cm−1 (B3LYP value; Table 1S, Supporting Information) of the lowest bending vibration. The vibration−rotation constants calculated from the entries in Table 3 are αA = −38.64(47), αB = −1.666(25), and αC = 3.184(32) MHz, in satisfactory agreement with the B3LYP values that are −34.26, −3.41, and 1.90 MHz, respectively (Table 1S, Supporting Information). The assignments referred to above include the vast majority of the strongest lines and lines of intermediate intensity. The remaining weaker unassigned lines are assumed to belong to further vibrationally excited states. There is no evidence of the existence of additional conformers. Energy Difference Between ap and sc. The internal energy difference between these two forms was obtained from comparison of the intensities of several selected transitions of the ground states of the two conformers using the procedure outlined by Esbitt and Wilson.59 The dipole moment must be known in order to derive the energy difference. The experimental dipole moment of ap is not available. The CCSD dipole moments (Table 1) of the two forms were therefore used. The result was Eap − Esc = 0.2(7) kJ/mol, which means that sc tends to be slightly more stable than ap by 0.2 kJ/mol. However, the estimated uncertainty of ±0.7 kJ/mol is so large that the opposite cannot be excluded. It is concluded that ap and sc have nearly the same energy. The experimental value is in excellent agreement with the CCSD result (0.16 kJ/mol) and in good agreement with the B3LYP result (1.86 kJ/mol).
Polarity of the substituents seems to be important. The dipole moment of the sc form of the isoelectronic compound C3H5CH2CCH is 2.52(3) × 10−30 C m,33 whereas its ap rotamer has a dipole moment of 2.13(3) × 10−30 C m,33 much less than roughly 12 × 10−30 C m found for the two forms of the title compound (Tables 1 and 3). The ap form is favored 0.77(36) kJ/mol in C3H5CH2CCH,33 while sc is favored by 0.2(7) kJ/mol in C3H5CH2NC. The corresponding nitrile C3H5CH2CN, which must have a dipole moment similar to C3H5CH2NC, prefers a sc form.31 The preference of the nitrile and the isonitrile to take sc conformations is in accord with the gauche effect,35 which predicts that electronegative substituents prefer sc conformations. Steric repulsive forces may also play a role in both conformers of cyclopropylmethyl isocyanide. The C1−C2−C9 and C3−C2− C9 angles are the same by symmetry and as large as 121.4° in ap according to the CCSD calculations (Table 1). This is about 4−5° larger than the C−C−H angles of the cyclopropyl ring and about 3° larger than the two angles in sc. The opening up of the C1−C2−C9 and C3−C2−C9 angles seen in ap, but not in sc, could reflect a repulsion between the cyclopropyl ring and the isocyano group in the former rotamer. Another repulsion seems to be present in sc. In the CCSD calculations (Table 1), the H6−C2−C9−N12 dihedral angle (Table 1) is calculated to be 63.2°, three degrees lager than the canonical 60° angle possibly because of repulsion between the pseudo-π electrons along the edge of the cyclopropyl ring36 and the π-electrons of the isocyano group. The observed energy difference of Eap − Esc = 0.2(7) kJ/mol must therefore be a compromise of several intramolecular forces.
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ASSOCIATED CONTENT
S Supporting Information *
DISCUSSION The conformational properties of cyclopropylmethyl isocyanide are presumably dictated by several intramolecular interactions.
Results of the theoretical calculations, including electronic energies; molecular structures; dipole moments; harmonic and G
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anharmonic vibrational frequencies; rotational and centrifugal distortion constants; rotation−vibration constants; and 14N nuclear quadrupole coupling constants. Microwave spectra of the ground and vibrationally excited states. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(H.M.) Tel: +47 2285 5674. Fax: +47 2285 5441. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Anne Horn for her skillful assistance. This work has been supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/V30). It has also received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant No. NN4654K). J.-C.G. thanks the Centre National of Space Science (CNES) and the program PCMI (INSU-CNRS) for financial support.
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