Microwave Spectrum and Conformational Composition of

Publication Date (Web): August 1, 2014 ... constants obtained in the CCSD calculations are in good agreement with the experimental rotational constant...
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Microwave Spectrum and Conformational Composition of (Azidomethyl)cyclopropane (C3H5CH2N3) Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡ †

Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, Blindern, NO-0315 Oslo, Norway ‡ Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France S Supporting Information *

ABSTRACT: The microwave spectrum of (azidomethyl)cyclopropane, C3H5CH2N3, has been investigated in the 26−90 GHz spectral range at a temperature of about −30 °C. Five rotameric forms of this compound, whose spectra can be distinguished by microwave spectroscopy, may exist. The spectra of three of them denoted III, IV, and V were assigned. The ground vibrational state spectra of III and V were assigned, while the ground and six vibrationally excited states were assigned for IV. These three rotamers all have a synclinal orientation of the H−C−C−N chain of atoms, while the C−C−N−N link is either + or − synclinal or antiperiplanar. Conformer IV, having synclinal orientation of the two said dihedral angles, was found to have the lowest energy by relative intensity measurements. Rotamer V has an energy that is 1.6(6) kJ/mol higher than the energy of IV, while the energy of III is 2.1(6) kJ/mol higher than the energy of IV. Quantum chemical calculations were performed at the MP2/cc-pVTZ and CCSD/cc-pVTZ levels of theory. The rotational constants obtained in the CCSD calculations are in good agreement with the experimental rotational constants, while the MP2 centrifugal distortion constants are generally in poorer agreement with their experimental counterparts.

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INTRODUCTION The properties of small organic azides (R-N3) have received relatively little attention in the past. The main reason for this is presumably their toxicity and the fact that some of them are explosive and should be treated with utmost care. However, recent studies1−4 have shown that azides have a number of useful synthetic properties. The structures and conformational behavior of gaseous organic azides should therefore be of interest to better understand the chemistry of this class of compounds. Roughly 25 years ago, Klaeboe, Nielsen, Priebe, Schei, and co-workers synthesized several small organic azides and performed studies of their structural and conformational properties by electron diffraction, infrared spectroscopy, and theoretical calculations, as summarized in a review.5 It was established in these investigations that gaseous azides indeed have unique conformational and structural properties.5 Very few conformational studies of azides have been reported in the intervening period. However, one year ago, we presented a novel synthesis of 2-fluoroethyl azide, FCH2CH2N3, and investigated its conformational properties by MW spectroscopy augmented with high-level quantum chemical calculations and found that one rotameric form predominates in this case.6 In the present work, our azide studies are extended to include the first microwave (MW) investigation of (azidomethyl)cyclopropane, C3H5CH2N3. This compound has two internal axes, namely, the C3H5− CH2N3 axis and the C3H5CH2−N3 axis, about which rotational isomerism may arise. Nine conformers are possible for this molecule, but only five conformers can be distinguished by means of their MW spectra. These five forms are shown in © 2014 American Chemical Society

Figure 1 and given Roman numeral for reference. Confomer I is unique, whereas mirror-image forms exist for the remaining four rotamers. The enantiomers of II−V, which have identical moments of inertia and dipole moments and identical MW spectra, can be obtained by proper sign changes of relevant dihedral angles. Atom numbering is shown in Figure 1 on rotamer I. The H6−C2−C9−N10 and the C2−C9−N10−N13 dihedral angles can conveniently be used to characterize these five forms. The H6−C2−C9−N10 link of atoms is exactly antiperiplanar (ap) in I. This is also the case for the C2−C9−N10−N13 chain. This form has a symmetry plane bisecting the cyclopropyl ring. The former atomic arrangement is ap in II, while the C2−C9− N10−N13 atoms have a +synclinal (+sc) arrangement in this rotamer. Conformers III−V all have a +sc conformation of the H6−C2−C9−N10 chain, whereas C2−C9−N10−N13 atoms are ap in III, +sc in IV, and −sc in V. MW spectroscopy is an ideal experimental method to study the complex conformational problems such as the one associated with the title compound, due to its extreme accuracy and resolution. The fact that accurate energy differences can be obtained from relative intensity measurements is another advantage of this method. The MW work has been augmented with high-level quantum chemical calculations to investigate properties of the conformational hypersurface and to obtain information that is useful for the assignment of the spectrum. Received: June 24, 2014 Revised: July 29, 2014 Published: August 1, 2014 6971

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compounds were distilled at 0.1 mbar at room temperature using a vacuum line. A first trap cooled at −20 °C removed some impurities, and the product was condensed in a second trap cooled at −70 °C. The (azidomethyl)cyclopropane was thus obtained in pure form in a 77% yield (1.87 g, 19.2 mmol). 1 H NMR (CDCl3) δ 0.25 (m, 4H, 3JHH = 6.0 Hz, 2 CH2 of the cycle); 1.09 (m, 1H, 3JHH = 6.0 Hz, 3JHH = 7.1 Hz, CH); 3.07 (d, 2H, 3JHH = 7.1 Hz, CH2N3). 13C NMR (CDCl3) δ 3.3 (1JCH = 162.8 Hz (t), CH2 cycle); 10.0 (1JCH = 162.1 Hz (d), CH cycle); 56.0 (1JCH = 142.3 Hz (t), CH2N3). Spectroscopic Experiments. (Azidomethyl)cyclopropane is a colorless liquid with a vapor pressure of roughly 110 Pa at room temperature. Its MW spectrum was recorded with the cell cooled to about −30 °C using small portions of dry ice to cool the waveguide. The pressure was 5−10 Pa during the measurements. The MW spectrum was studied with Starkmodulation spectroscopy using the microwave spectrometer of the University of Oslo. Details of the construction and operation of this device have been given elsewhere.8 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 26−90 GHz frequency interval. Radio-frequency microwave double-resonance experiments (RFMWDR), similar to those of Wodarczyk and Wilson,9 were also conducted to unambiguously assign particular transitions, using the equipment described elsewhere.8

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RESULTS Quantum Chemical Calculations. The present frozencore MP210 and CCSD calculations were performed employing the Gaussian0911 and Molpro12 programs running on the Abel cluster in Oslo using Dunning’s13 correlation-consistent ccpVTZ basis set, which is of triple-ζ quality. The default convergence criteria of the two computer programs were used. MP2/cc-pVTZ computations of the structures, dipole moments, vibrational frequencies, and Watson’s A-reduction quartic centrifugal distortion constants14 were performed for the five conformers I−V. All structural parameters were varied freely in these calculations with no symmetry restrictions. No imaginary harmonic normal vibrations were obtained, which is an indication that these rotamers are minima on the potential energy hypersurface. The precautions of McKean et al.15 were observed in the computations of the centrifugal distortion constants. Selected results of these calculations are found in Tables 1S−5S in the Supporting Information. The comprehensive CCSD/cc-pVTZ calculations of optimized structures, dipole moments, and electronic energies of forms I−V were performed using the MP2 structures in Tables 1S−5S, Supporting Information, as starting points. The optimizations were done without symmetry restrictions. The resulting structures are listed in Table 1, while additional structural details are listed in Tables 6S−10S of the Supporting Information. The rotational constants calculated from these structures are shown in Table 2. The dipole moment components of the Molpro calculations were transferred to the principal-axes dipole moment components with the results listed in Table 2. Calculation of the centrifugal distortion constants by the CCSD method is beyond our computational resources. The MP2/cc-pVTZ quartic centrifugal distortion constants in the S-reduction form14 are included in Table 2. Some of the results in Tables 1 and 2 warrant further comments. The geometry of the azido group is interesting. The

Figure 1. Five rotameric forms of (azidomethyl)cyclopropane. Atom numbering is indicated on conformer I.

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EXPERIMENTAL SECTION Synthesis. Caution! (Azidomethyl)cyclopropane is potentially toxic and explosive and all experiments should be performed behind a safety screen and in a well-ventilated hood. The synthesis of (azidomethyl)cyclopropane has already been reported but only few details were given.7 Since small azides are potentially explosive on heating, we applied the procedure we have already reported for 2-fluoroethyl azide6 using a high boiling solvent. This procedure allows the purification of the expected product by room temperature distillation since it is the sole low boiling compound of the mixture when the reaction is completed. (Scheme 1). Scheme 1

A stirred suspension of sodium azide (3.25 g, 50 mmol) in triethylene glycol (50 mL) was degassed under vacuum (0.1 mbar) for 10 min at room temperature. The (bromomethyl)cyclopropane (3.38 g, 25 mmol) was then added, and the resulting mixture was stirred under nitrogen at 50 °C for 20 h. After checking of the complete transformation of the precursor by 1H NMR spectroscopy on a small sample, the low-boiling 6972

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Table 1. CCSD/cc-pVTZ Structures of Five Conformers of C3H5CH2N3a I C1−C2 C1−C3 C1−H4 C1−H5 C2−C3 C2−H6 C2−C9 C3−H7 C3−H8 C9−N10 C9−H11 C9−H12 N10−N13 N13−N14

150.2 151.0 108.0 108.0 150.2 108.2 150.7 108.0 108.0 147.8 109.4 109.4 123.2 112.9

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−N10 C2−C9−H11 C2−C9−H12 N10−C9−H11 N10−C9−H12 H11−C9−H12 C9−N10−N13 N10−N13−N14

117.9 117.3 118.2 116.9 115.4 116.2 121.1 116.2 121.1 112.6 118.2 116.9 117.9 117.3 115.4 109.6 109.8 109.8 109.7 109.7 108.1 113.0 175.3b

H4−C1−C2−H6 H4−C1−C2−C9 H5−C1−C2−H6 H5−C1−C2−C9 H4−C1−C3−H7 H4−C1−C3−H8 H5−C1−C3−H7 H5−C1−C3−H8 H6−C2−C3−H7 H6−C2−C3−H8 C9−C2−C3−H7 C9−C2−C3−H8 C1−C2−C9−N10 C1−C2−C9−H11 C1−C2−C9−H12 C3−C2−C9−N10 C3−C2−C9−H11 C3−C2−C9−H12 H6−C2−C9−N10 H6−C2−C9−H11 H6−C2−C9−H12

−1.5 141.3 −146.6 −3.8 0.0 −145.0 145.0 0.0 1.5 146.6 −141.3 3.8 35.9 −84.7 156.5 −35.9 −156.5 84.7 180.0 59.4 −59.4

II Bond Length (pm) 150.2 150.9 108.0 108.0 150.3 108.3 151.2 108.0 108.0 147.8 108.9 109.3 123.4 112.9 Angles (deg) 118.0 117.2 118.2 116.9 115.4 116.1 120.9 115.9 121.6 112.7 118.3 117.1 117.8 118.3 114.7 112.9 109.7 110.2 105.7 110.3 107.8 113.5 176.4b Dihedral Angle (deg) −1.8 140.8 −146.9 −4.3 0.4 −143.6 145.2 1.3 1.6 146.8 −141.6 3.5 35.1 −82.5 158.9 −36.9 −154.5 87.0 178.8 61.2 −57.4 6973

III

IV

V

150.1 150.8 107.9 108.1 150.4 108.1 149.9 107.9 108.1 148.1 109.4 109.4 123.3 112.9

150.7 150.7 108.0 108.1 150.4 108.2 150.5 107.9 108.1 147.9 108.7 109.5 123.5 112.9

150.2 150.7 108.0 108.1 150.5 108.3 150.5 108.0 108.1 148.1 109.4 108.8 123.4 112.9

118.1 117.2 118.1 117.8 114.8 116.7 118.9 116.9 118.6 114.7 118.2 117.7 118.0 117.7 114.7 108.8 110.6 109.6 109.9 109.9 108.0 112.9 175.3b

118.1 117.7 118.2 117.5 114.7 116.5 119.5 116.7 118.9 114.6 118.2 117.6 118.0 117.6 114.7 113.4 110.8 109.8 104.7 110.2 107.7 113.3 174.5b

118.2 117.1 118.0 117.9 114.8 116.6 118.9 116.3 118.7 115.2 118.2 117.8 118.1 117.5 114.7 113.4 110.8 109.8 109.8 105.0 107.7 112.9 175.3b

−0.7 143.7 −144.8 −0.3 0.2 −144.5 145.3 0.5 1.2 145.5 −143.1 1.2 −82.3 156.8 37.9 −152.1 87.0 −31.9 62.8 −58.1 −177.0

−1.1 143.6 −145.7 −1.0 0.1 −144.5 144.5 −0.1 1.4 145.7 −142.5 1.8 −88.3 154.3 35.5 −158.2 84.4 −34.4 57.1 −60.3 −179.1

−1.3 143.8 −145.4 −0.2 0.3 −144.6 145.3 0.4 1.1 145.3 −143.2 1.0 −78.7 157.3 38.4 −148.5 87.5 −31.4 66.8 −57.1 −176.1

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Table 1. continued I C2−C9−N10−N13 H11−C9−N10−N13 H12−C9−N10−N13 a

II Dihedral Angle (deg) 95.0 −145.0 −28.7

180.0 −59.3 59.3

III

IV

V

−174.3 −53.0 65.7

64.0 −175.1 −59.5

−65.5 59.0 174.6

The structures of the three conformers whose MW spectra were assigned are given in boldface. bN14 bent away from C9.

unusual value of this angle is presumably caused by steric repulsion between the azido group and the cyclopropyl ring. The nonbonded distance between H5 and N10 is 263.5 pm, the nonbonded length between H8 and N10 is 270.1 pm (Table 7S, Supporting Information), while the distance between H8 and N14 is 262.4 pm. The sum of the Pauling van der Waals radii17 of hydrogen (120 pm) and nitrogen (150 pm) is 270 pm. A smaller value than 95.0° would have brought the said atoms into even closer proximity with steric repulsion as a consequence. The CCSD electronic energy differences between the various conformers that are listed in Table 2 indicates that conformer IV is the most stable form of the molecule, being more stable than I, II, III, and V by 3.26, 3.89, 2.86, and 1.98 kJ/mol, respectively. The corresponding MP2/cc-pVTZ energy differences corrected for harmonic zero-point vibrational energies can be obtained from entries in Tables 1S−5S of the Supporting Information as 3.81, 4.36, 3.98, and 2.49 kJ/mol, respectively. Microwave Spectrum and Assignment of Conformer IV. The CCSD and MP2 calculations indicate that the energy differences between the five forms of (azidomethyl)cyclopropane are a few kJ/mol, with rotamer IV as the lowest-energy conformer. The MP2 computations of the harmonic vibrational fundamental frequencies of rotamer IV (Supporting Information; Table 4S) indicate that there are six normal vibrations with frequencies below 500 cm−1. Similar results were found for the four other rotamers (Table 1S−3S and 5S, Supporting Information). The corresponding Boltzmann factors of these low-frequency vibrationally excited states of each rotamer are therefore relatively large and the ground vibrational state of each conformer is predicted to be accompanied by a number of prominent satellite lines belonging to the vibrationally excited state. All this should result in a very rich and comparatively weak spectrum at −30 °C. The observed spectrum is relatively weak with absorption lines occurring every few MHz throughout the entire MW region (26−90 GHz) in accord with expectations. The a-type R-branch transitions of conformer IV were first searched for because this conformer has its major dipole moment component along the a-inertial axis (Table 2). The CCSD rotational and the MP2 centrifugal distortion constants (Table 2) were used to predict the transition frequencies of this spectrum. Ray’s asymmetry parameter18 κ is −0.834, and the high-K−1 aR-transitions will therefore have rapid Stark effects due to their near-degeneracy. Searches for these lines were conducted employing a Stark field strength of about 110 V/cm. These transitions turned out to be the strongest ones in the spectrum under these experimental conditions, and they were readily assigned. The assignments were confirmed by RFMWDR experiments9 and fit to Watson’s S-reduction Irrepresentation Hamiltonian14 using Sørensen’s least-squares program Rotfit.19 The assignments were gradually extended to

Table 2. CCSD/cc-pVTZ and MP2/cc-pVTZ Parameters of Spectroscopic Interesta of Five Conformers of C3H5CH2N3 I A B C DJ DJK DK d1 d2 μa μb μc μtot

II

III

IV

Rotational Constants (MHz) 9013.7 5715.5 11838.5 5776.8 1362.3 1815.8 1184.4 1828.0 1316.7 1610.1 1142.7 1485.1 Quartic Centrifugal Distortion Constantsb (kHz) 0.150 1.57 0.168 1.28 2.62 −11.0 −4.76 −6.74 9.35 30.7 163 14.9 −0.0123 −0.436 0.00891 −0.402 0.00252 −0.0172 −0.00126 −0.0212 Dipole Moment (Debye) 1.29 1.55 1.83 2.14 1.41 0.75 1.02 0.26 0.0c 1.09 0.22 0.39 1.91 2.04 2.10 2.19 Energy Differenced (kJ/mol) 3.26 3.89 2.86 0.0

V 7240.5 1461.7 1392.7 1.66 −20.9 102 −0.436 −0.0214 2.00 0.27 0.85 2.19 1.98

a

CCSD rotational constants, dipole moments, and energy differences; MP2 centrifugal distortion constants. The theoretical parameters of the three conformers whose MW spectra were assigned are given in boldface. bS-reduction Ir-representation.14 cBy symmetry. dRelative to conformer I.

N10−N13 and N13−N14 bond lengths vary little from one conformer to the next. The N10−N13 bond is longer than the N13−N14 bond by more than 10 pm (Table 1), which is consistent with a resonance hybrid description of the azido group where the C3H5CH2−NN+N− and C3H5CH2−N−− N+N resonance structures are the main contributors. The rs length of the longer N−N bond in CH3N3 is 123.1 pm, while the terminal N−N bond length is 113.7 pm.16 The first of these is slightly shorter by 0.1−0.4 pm than the N10−N13 bond lengths of (azidomethyl)cyclopropane, while the second is 0.8 pm longer. The azido group of I−V is nonlinear with N14 bent away from C9. The deviations from linearity vary from 3.6° (conformer II) to 5.5° (IV), compared to 6.9° for this group in CH3N3.16 The C2−C9−N10 angle are 3−5° smaller when the C2−C9−N10−N13 link of atoms has a ap conformation (rotamers I and III) than a ±sc conformation (II, IV, and V), which may be due to repulsion between the azido group and the ring or a slight rehybridization of the C9 atom in the sc forms. The H6−C2−C9−N10 dihedral angle is exactly 180° in I and 178.8° in II and between 57.1° and 66.8° in the three other conformers. The C2−C9−N10−N13 dihedral angle is exactly ap (180°) in I and −174.3° in III. An interesting variation is seen when this atomic arrangement has a +sc or a −sc conformation. In IV this angle has a “normal” value of 64.0°. The same is true for V (−65.5°). However, this dihedral angle is as large as 95.0° instead of the expected ∼60° in II. The 6974

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Table 3. Spectroscopic Constantsa of the Ground and Vibrationally Excited States of Conformer IV of C3H5CH2N3 vib. state

ground state

first ex. tors.

second ex. tors.

third ex. tors.

fourth ex. tors.

first ex. lowest bend.

combination mode

Av (MHz) Bv (MHz) Cv (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJK (Hz) HKJ(Hz) rmsc Nd

5719.702(65) 1831.3867(21) 1481.6329(18) 1.44168(76) −8.0232(72) 14.85b −0.46243(81) −0.01840(71) 0.0549(71) −0.462(14) 1.201 450

5734.393(69) 1823.3073(21) 1480.4264(18) 1.49671(88) −8.547(10) 14.85b −0.47229(82) −0.01677(63) 0.393(85) −0.369(35) 1.314 427

5748.103(84) 1816.0608(25) 1479.2079(22) 1.55672(85) −9.0968(89) 14.85b −0.48728(88) −0.01425(72) 0.0932(86) −0.316(11) 1.230 372

5759.44(15) 1809.6902(29) 1478.1982(22) 1.6264(13) −9.764(14) 14.85b −0.5008(12) −0.0160(11) 0.145(13) −0.886(56) 1.419 300

5713.6(28) 1803.383(51) 1478.828(50) 1.6779(32) −10.177(12) 14.85b −0.496(13) −0.145(25) 0.0b 0.0b 1.233 150

5769.135(71) 1819.0341(21) 1474.1330(20) 1.42599(61) −8.3098(73) 14.85b −0.46311(87) −0.02096(72) 0.0b −0.166(25) 1.293 366

5788.03(10) 1810.0239(25) 1473.2268(24) 1.51026(72) −9.152(11) 14.85b −0.4793(10) −0.01986(88) 0.0 −0.304(47) 1.392 328

a

S-reduction Ir-representation.14 Uncertainties represent one standard deviation. The spectra are found in Tables 11S−17S of the Supporting Information. bFixed at this value in the least-squares fit. cRoot-mean-square deviation defined as rms2 = Σ[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the leastsquares fit, and P is the number of spectroscopic constants used in the fit. dNumber of transitions used in the fit.

strongest of these satellite spectra is assumed to belong to the first excited state of the torsional vibration. Relative intensity measurements yielded 56(15) cm−1 for this mode compared to the MP2 value of 62 cm−1 (Table 4S, Supporting Information). The spectra of a total of four successively excited states (Table 12S−15S, Supporting Information) of this mode were assigned with the resulting spectroscopic constants listed in Table 3. It is seen from this table that the variation of the rotational constants upon successive excitation is nearly constant for the first three excited states of the torsion, whereas deviations from this pattern are observed for the fourth excited state of this mode. This deviation could be due to interactions with other vibrational modes. The smooth variation of the rotational constants of the first three excited states is typical for a harmonic vibration.20 The 366 transitions of the spectrum of the lowest bending vibration are listed in Table 16S, Supporting Information, while the spectroscopic constants are given in Table 3. The MP2 frequency of this mode is 95 cm−1 (Table 4S, Supporting Information). Relative intensity measurements yielded 89(20) cm−1 for this vibration. The last excited state of Table 3 is assumed to be a combination mode consisting of the first excited state of the torsion plus the first excited state of the lowest bending vibration. The changes of the rotational constants from the first excited state of the lowest bending vibration to this combination state is almost, but not exactly, the same as the changes from the rotational constants of the ground vibrational state to the first excited state of the torsion. Its frequency was determined to be 145(30) cm−1 from relative intensity measurements. Assignment of the Spectrum of V. This rotamer is calculated to be 1.98 kJ/mol higher in energy than IV (Table 2) and has its major dipole moment component of 2.00 D along the a-axis. The Ray asymmetry parameter κ is −0.969, and characteristic pile-up regions of aR-transitions separated by approximately the sum of the B and C rotational constants are predicted. The intermediate and high-K−1 members of these series are modulated at low Stark fields. These properties were exploited to assign the spectrum of this conformer. Typical series of a-type transitions, such as the J = 17 ← 16 transitions shown in Figure 2, were first identified. The assignments of the a R-lines were gradually extended to include values up to Jmax =

include higher and higher values of the principal quantum number J and the lower K−1 transitions. Ultimately, 450 aRtransitions with Jmax = 29 and K−1,max = 24 shown in Table 11S of the Supporting Information were assigned and used to determine the spectroscopic constants shown in Table 3. The quartic centrifugal distortion constant DK cannot be obtained from this selection of transitions and were preset at MP2 value (14.85 kHz). Two sextic centrifugal distortion constants, HJK and HKJ, were fitted with the remaining sextic constants preset at zero in the least-squares fit. Searches for b- and c-type lines were made, but none were found. This is not surprising since μb and μc are much smaller than μa (Table 2) resulting in insufficient intensity of the b- and c-type lines. The experimental rotational constants (Table 3) agree well with the CCSD rotational constants (Table 2). The deviations are −0.99% for A, 0.16% for B, and −0.23% for C. Differences of this order of magnitude are to be expected because of the different definitions of the rotational constants. The CCSD constants are derived from an approximate equilibrium structure, while the experimental rotational constants are effective constants. The good agreement indicates that the CCSD structure in Table 1 is indeed accurate, which was also expected for computations at the high CCSD/cc-pVTZ level. Much poorer agreement is seen for the experimental quartic centrifugal distortion constants and their MP2 counterparts. The best agreement is seen for DJ (+11.1%) and the worst for DJK (−16.0%). Obviously, MP2/cc-pVTZ calculations are not sufficient to predict accurate values for the quartic centrifugal distortion constants in the present case. Vibrationally Excited States of IV. The ground-state transitions of conformer IV were accompanied by a series of weaker transitions with very similar Stark effects and RFMWDR patterns. These lines were considered to belong to vibrationally excited states. The harmonic frequencies of the lowest vibrational fundamentals are 62, 95, 213, 240, 356, and 451 cm−1 according to the MP2 calculations (Table 4S, Supporting Information). The first of these fundamental vibrations (62 cm−1) is the torsion about the C9−N10 bond, while the other five fundamentals can best be described as bending vibrations. It is seen from Table 3 that the spectra of six vibrationally excited states have been assigned. The assignment procedure was similar to that of the ground vibrational state. The 6975

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2 shows that the experimental A rotational constant is 2.83% smaller than its theoretical counterpart, while the experimental values of B and C are 1.66% and 0.33% larger than predicted by theory. The experimental MP2 quartic centrifugal distortion constants (Table 4) are all significantly larger than their theoretical equivalents (Table 2). Assignment of the Spectrum of III. This rotamer is calculated to be 2.86 kJ/mol higher in energy than the energy of the global-minimum form IV (Table 2). Its asymmetry parameter κ is −0.992, and μa is the predominating dipole moment component (Table 2). A pile-up spectrum, whose features are very similar to those described for conformer V, was predicted and soon identified. The J = 28 ← 27 a-type spectrum is shown in Figure 3. The spectrum of III is even

Figure 2. Portion of the J = 17 ← 16 a-type transitions of conformer V. The spectrum was taken at a field strength of about 110 V/cm. Values of the K−1 pseudo quantum number are listed above several peaks belonging to V. The transitions with K−1 quantum numbers 5,6, 3,4, and 3,12 are not resolved. The remaining unlabeled transitions are presumed to belong to vibrationally excited states of V or to further rotameric forms. The intensity is in arbitrary units.

31 and K−1max = 22. The c-dipole moment component is the second largest component according to Table 2. Searches for ctype lines were made, but none could be unambiguously assigned presumably due to their weakness and the spectral richness with very frequent overlapping lines. The 407 aRtransitions used to determine the spectroscopic constants in Table 4 are listed in Table 18S, Supporting Information. The Figure 3. J = 28 ← 27 aR-transitions of conformer III. The spectrum was recorded at a Stark field strength of about 110 V/cm. Values of the K−1 pseudo quantum number are listed above several peaks belonging to III. The transitions with K−1 quantum numbers 5, 6, and 7 are not resolved. The remaining unlabeled transitions are presumed to belong to vibrationally excited states of III or to other conformers. The intensity is in arbitrary units.

Table 4. Spectroscopic Constantsa of the Ground Vibrational State of Conformer V of C3H5CH2N3 Av (MHz) Bv (MHz) Cv (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJ (Hz) HJK (Hz) HKJ(Hz) rmsa Na

7035.0(17) 1485.9842(78) 1397.6745(81) 2.3199(20) −31.1213(67) 102a −0.6590(44) −0.0257(19) 0.0274(12) −0.2202(51) −1.841(16) 1.238 407

weaker than the spectrum of conformer V. The assignments of 320 transitions (Table 19S, Supporting Information) were made for values of J and K−1 up to 38 and 24, respectively, in the same way as described in the previous paragraph. The K−1 = 1 or 0 are weak and difficult to modulate and frequently overlapped, and definite assignments of these transitions could not be made. The rotational constants, DJ, DJK, d1, HJK, and HKJ, were fitted, while DK and d2 could not be determined from these aR-lines. DK and d2 were fixed at the MP2 value (Table 2) in the least-squares fit, whereas further sextic constants were preset at zero. No vibrationally excited-state spectrum could be assigned for intensity reasons. The experimental spectroscopic constants are displayed in Table 5. The experimental rotational constants of this table deviate by less than 0.6% from their CCSD counterparts. Much larger differences are seen for the quartic centrifugal distortion constants. Searches for Further Conformers. Rotamers I and II are predicted to be the highest-energy forms of (azidomethyl)cyclopropane with an energy difference of 3−4 kJ/mol relative to the global-minimum conformer IV (Table 2). Form I has a statistical weight of 1 vis-à-vis the four other conformers (II− V) whose statistical weight is 2 relative to I. The assignment of

a

Footnotes are the same as those given for Table 3. The spectrum is given in Table 18S of the Supporting Information.

rotational constants, four quartic centrifugal distortion constants, DJ, DJK, d1, and d2, as well as three sextic centrifugal distortion constants, HJ, HJK, and HKJ, were fitted. DK could not be determined from these aR-lines and was fixed at the MP2 value (102 kHz; Table 2). Further sextic constants were preset at zero in the least-squares fit. No excited-state spectra of this conformer could be unambiguously assigned due to their weakness. Comparison of the experimental rotational constants listed in Table 4 and the CCSD rotational constants displayed in Table 6976

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Table 5. Spectroscopic Constantsa of the Ground Vibrational State of Conformer III of C3H5CH2N3 Av (MHz) Bv (MHz) Cv (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJK (Hz) HKJ(Hz) rmsa Na

possible that this results in steric repulsion and may therefore be part of the explanation why IV is preferred over V. The C2−C9−N10−N13 chain is antiperiplanar in III, whose energy is +2.1(6) kJ/mol relative to IV. Steric repulsion is absent in this rotamer, but this is not sufficient to make this conformer the global minimum-energy form. One reason for this could be the highly electronegative nature of the nitrogen atom whose Pauling electronegativity value is 3.04.24 The socalled gauche effect25 predicts that sc forms are preferred for highly electronegative atoms or groups of atoms, and this may have played a role in the present case.

11897(44) 1177.057(38) 1136.174(38) 0.18116(46) −5.5982(57) 163a 0.0103(44) 0.000126a −0.0570(24) 1.5030(83) 1.331 320

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ASSOCIATED CONTENT

S Supporting Information *

Results of the theoretical calculations, including electronic energies; molecular structures; dipole moments; harmonic vibrational frequencies; and rotational and centrifugal distortion constants. Microwave spectra of the ground and vibrationally excited states of three conformers. This material is available free of charge via the Internet at http://pubs.acs.org.

a

Footnotes are the same as those given for Table 3. The spectrum is given in Table 19S in the Supporting Information.

the spectrum of I was therefore considered to be very demanding in this weak spectrum. Searches were nevertheless performed using Stark and RFMWDR spectroscopies, but with negative outcome. The chances to assign the spectrum of II looked a little better, and considerable efforts were made toward this end. This conformer was predicted (Table 2) to have an energy that is about 1 kJ/mol higher than the energy of III, which has the weakest of the three assigned spectra. In addition, II has dipole moment components, which are significantly lower than μa of III (Table 2). All this can explain why the very weak spectrum of this form was not found despite extensive Stark and RFMWDR searches. Energy Differences. The energy differences between the three identified conformers were determined by relative intensity measurements21 performed on relatively strong, fully modulated absorption lines observing the precautions of Esbitt and Wilson.22 The intensity and thereby the energy differences depend on the dipole moment component of the transitions in question, and the theoretical values given in Table 2 were employed in the energy-difference computations. Conformer IV was found to be the global minimum being 1.6(6) kJ/mol lower in energy than V, and 2.1(6) kJ/mol lower in energy than III. The uncertainties are estimated to one standard deviation. The corresponding CCSD values (Table 2) were 1.98 and 2.86 kJ/mol, respectively. The experimental values tend to be slightly less than the CCSD predictions.

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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 d’Etudes Spatiales (CNES) for financial support.

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REFERENCES

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DISCUSSION (Azidomethyl)cyclopropane displays a complex conformational equilibrium consisting of at least three rotamers III, IV, and V. There are probably several reasons why these three forms are preferred over I and II. The identified forms III, IV, and V have one thing in common, namely, a synclinal orientation of the H6−C2−C9−N10 chain of atoms. This kind of sc preference is typical for monosubstituted methylcyclopropanes (C3H5CH2X) and displayed by the vast majority of these compounds.23 The sc inclination seems to be almost independent of the nature of the X substituent. The lowest-energy conformer IV and the second lowestenergy rotamer V (+1.6(6) kJ/mol) both have sc orientation for the C2−C9−N10−N13 link of atoms. There are no close nonbonded contacts between N13 and atoms of the cyclopropyl ring in IV (Table 9S, Supporting Information), while a close contact of 281 pm exists between N13 and H6 in V. It is 6977

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(10) Møller, C.; Plesset, M. S. Note on the Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision B.01; Gaussian, Inc: Wallingford, CT, 2010. (12) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; et al. MOLPRO, version 2010.1, a Package of Ab Initio Programs; Cardiff University: Cardiff, U.K., 2010. http://www.molpro.net/. (13) Peterson, K. A.; Dunning, T. H., Jr. Accurate Correlation Consistent Basis Sets for Molecular Core-Valence Correlation Effects: The Second Row Atoms Al-Ar, and the First Row Atoms B-Ne Revisited. J. Chem. Phys. 2002, 117, 10548−10560. (14) Watson, J. K. G. Vibrational Spectra and Structure; Elsevier: Amsterdam, The Netherlands, 1977; Vol. 6. (15) McKean, D. C.; Craig, N. C.; Law, M. M. Scaled Quantum Chemical Force Fields for 1,1-Difluorocyclopropane and the Influence of Vibrational Anharmonicity. J. Phys. Chem. A 2008, 112, 6760−6771. (16) Heineking, N.; Gerry, M. C. L. Nitrogen-14 Nuclear Quadrupole Hyperfine Structure in the Microwave Spectrum of Methyl Azide, CH3N3. Z. Naturforsch., A: Phys. Sci. 1989, 44, 669−674. (17) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960. (18) Ray, B. S. The Characteristic Values of an Asymmetric Top. Z. Phys. 1932, 78, 74−91. (19) Sørensen, G. O. Centrifugal Distortion Analysis of Microwave Spectra of Asymmetric Top Molecules. The Microwave Spectrum of Pyridine. J. Mol. Spectrosc. 1967, 22, 325−346. (20) Herschbach, D. R.; Laurie, V. W. Influence of Vibrations on Molecular Structure Determinations. I. General Formulation of Vibration-Rotation Interactions. J. Chem. Phys. 1962, 37, 1668−1686. (21) Townes, C. H.; Schawlow, A. L. Microwave Spectroscopy; McGraw-Hill: New York, 1955. (22) Esbitt, A. S.; Wilson, E. B. Relative Intensity. Rev. Sci. Instrum. 1963, 34, 901−907. (23) Samdal, S.; Møllendal, H.; Guillemin, J.-C. Microwave Spectrum, Conformational Properties, and Dipole Moment of Cyclopropylmethyl Isocyanide (C3H5CH2NC). J. Phys. Chem. A 2013, 117, 5073−5081. (24) Allred, A. L. Electronegativity Values from Thermochemical Data. J. Inorg. Nucl. Chem. 1961, 17, 215−221. (25) Wolfe, S. Gauche Effect. Stereochemical Consequences of Adjacent Electron Pairs and Polar Bonds. Acc. Chem. Res. 1972, 5, 102−111.

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