Article pubs.acs.org/JPCA
Microwave Spectrum and Conformational Properties of 4‑Isocyano1-butene (H2CCHCH2CH2NC) Svein Samdal,† Terje Grønås,† 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, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France S Supporting Information *
ABSTRACT: The microwave spectrum of 4-isocyano-1-butene (H 2 C CHCH2CH2NC) has been investigated in the 35−80 GHz spectral region. Selected measurements have also been made outside this spectral range. Rotation about the −CH−CH2− and −CH2−CH2− single bonds may produce rotational isomerism resulting in five conformers. The spectra of three of them, denoted I, III, and IV, have been assigned. In conformer I, the CC−C−C link of atoms is +anticlinal and the C−C−C−N chain is antiperiplanar. In III, the two links of atoms are +anticlinal and +synclinal, whereas in IV, the two chains are synperiplanar and antiperiplanar, respectively. Conformer I was found to have the lowest energy of the three forms by relative intensity measurements. These measurements yielded EIII − EI = 1.1(7) kJ/mol, and EIV − EI = 2.9(7) kJ/mol for the internal energy differences. The microwave study was augmented by quantum chemical calculations at the CCSD/cc-pVQZ and MP2/cc-pVTZ levels of theory. Good agreement between experimental and theoretical results was seen in most cases.
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INTRODUCTION Isocyanides are an interesting class of compounds possessing a unique chemistry that has been investigated comparatively little.1−3 We have therefore synthesized several isocyanides, which have subsequently been investigated by UV photon electron spectroscopy,4 microwave (MW) spectroscopy,5−9 and high-level quantum chemical calculations.4−9 We have already reported MW spectra of allenyl isocyanide (H2CC CHNC),5 2-fluoroethyl isocyanide (FCH2CH2NC),6 2-chloroethyl isocyanide (ClCH2CH2NC),7 E- and Z-1-propenylisocyanide (CH3CHCHNC),8 cyclopropylmethyl isocyanide (C 3 H 5 CH 2 NC), 9 and 4-isocyano-1-butyne (HC CCH2CH2NC).10 Our MW studies of H2CCCHNC5 and E- and Z-CH3CHCHNC8 were undertaken because of their potential astrochemical interest, while conformational properties were the focus of our investigations of 2-fluoroethyl,6 2-chloroethyl,7 cyclopropylmethyl isocyanide,9 and 4-isocyano1-butyne.10 In this work, our studies are extended to include the first MW investigation of the structural and conformational properties of 4-isocyano-1-butene (H2CCHCH2CH2N C). This compound has two single bonds and rotational isomerism is therefore possible. Five typical conformers are depicted in Figure 1 and given Roman numerals for reference. Atom numbering is shown on I. The C1C2C3C4 and C2C3C4N5 dihedral angle can conveniently be used to describe the conformations of the five forms. The C1C2C3C4 dihedral angle is anticlinal (about 120° from © 2014 American Chemical Society
synperiplanar) in rotamers I−III, and synperiplanar (approximately 0°) in IV and V. The C2C3C4N5 dihedral angle is antiperiplanar (roughly 180°) in I and IV, −synclinal (about −60°) in II and V, and +synclinal (ca. 60°) in III. Mirror-image forms, which can be obtained by adequate changes of the signs of the dihedral angle(s), exist for all conformers but IV. 4-Isocyano-1-butene is a 4-substituted 1-butene, H2C CHCH2CH2X, where X is the substituent. The structural and conformational properties of this class of compound, which are analogous to that of H2CCHCH2CH2NC, have received considerable attention in the past. One example is 4-fluoro-1butene (H2CCHCH2CH2F), which has been studied several times.11−13 A conformer corresponding to III has been found to be the lowest energy form.11−13 Two additional forms with slightly higher energies were found in the gas-phase in a MW work,12 while all five forms were identified in a far-infrared study of a krypton solution.13 A gas electron diffraction (GED) and molecular-mechanics investigation of H 2 C CHCH2CH2Cl and H2CCHCH2CH2Br again showed that forms analogous to III are the most stable ones in these two cases and that there are minor contributions from other conformers.14 The preferred form of H2CCHCH2CH2OH corresponds to II, and it is stabilized by an intramolecular hydrogen bond between the hydroxyl group and the πReceived: December 13, 2013 Revised: January 27, 2014 Published: January 29, 2014 1413
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been investigated prompted the present research. Our experimental method of investigation is MW spectroscopy due to its superior accuracy and resolution, thereby making this method ideal for conformational and structural studies. The experimental work has been augmented by theoretical calculations, which were performed at a much higher level of theory than previously4 with the purpose of obtaining information for use in assigning the MW spectrum and of investigating properties of the potential-energy hypersurface.
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EXPERIMENTAL SECTION Synthesis. 4-Isocyano-1-butene was synthesized starting from the corresponding formamide as described previously (Scheme 1).4 A yield of 76% was obtained starting from 10 Scheme 1
mmol of N-3-buten-1-yl-formamide. 4-Isocyano-1-butene is a colorless liquid at room temperature with a vapor pressure of roughly 100 Pa. Spectrometer and Experimental Details. The Starkmodulated spectrometer of the University of Oslo was used in this study. This instrument has a resolution of about 0.5 MHz and measures the frequency of isolated transitions with an accuracy of 0.1 MHz. The spectrometer has a 50 kHz homebuilt Stark generator. The Stark cells are a 2-m-long HewlettPackard and homemade 2- and 3-m-long brass cells. The microwave radiation is generated using a 1730B Systron Donner frequency synthesizer operating in the 2−26.5 GHz frequency range. Several Millitech frequency multipliers are used to generate radiation in the 26.5−120 GHz spectral interval. The lock-in amplifier is a Perkin-Elmer model 5209. Double resonance radiofrequency microwave experiments (RFMWDR) similar to those performed by Wodarczyk and Wilson20 are performed using a Rohde & Schwarz SML01 signal generator operating in the 9 kHz to 1.1 GHz spectral region as the radio frequency source. An EIN Model 503L amplifier provides 3 W linear amplification of the radio signals between 2 and 510 MHz. Mixing of the radio signal with the Stark modulation signal is provided employing a HewlettPackard 10514 mixer. A National Instruments (NI) Compact RIO programmable automation controller (PAC) model cRIO9076 was synchronized with the 1730B Systron Donner Synthesizer for data acquisition. The PAC is a combination of a real-time controller, reconfigurable input/output modules (RIO), a FPGA module, and an Ethernet expansion chassis. The RIO moduls consists of a 24-Bit Analog Input Modul NI 9239 and a TTL digital moduls NI 9401. A program developed using LabVIEW runs on the PAC, triggers the synthesizer for the step sweep mode, and executes a high-speed data acquisition of multiple samples (16) for data averaging 12 ms after each trigger pulse. The data are buffered and streamed out to network. The LabVIEW FPGA Module makes it possible to program FPGA for custom synchronization and fast real time data decisions or analyses. The spectra were processed using the Grams/AI spectroscopy program.21 The MW spectrum of 4-isocyano-1-butene was recorded with the cell cooled to about −20 °C to enhance spectral
Figure 1. Five rotameric forms of H2CCHCH2CH2NC. Atom numbering is indicated on conformer I. The C1C2−C3−C4 link of atoms is ac in I−III, and sp in IV and V. The C2−C3−C4−N5 chain is ap in I and IV, −sc in II, and V +sc in III.
electrons of the double bond.15,16 The GED investigation15 indicates that more rotamers coexist with the hydrogen-bonded form. The MW spectra of three conformers of each of H2C CHCH2CH2SH17 and H2CCHCH2CH2SeH18 have been reported. The lowest-energy conformers of the thiol17 and selenol18 are stabilized by an internal hydrogen bond, just as in the case of H2CCHCH2CH2OH.15,16 The two additional higher energy rotamers of the thiol and selenol are similar to I and consequently not stabilized by this interaction. A total of four rotameric forms were assigned for H 2 C CHCH2CH2NH2 in a MW study.19 Two of these conformers are analogous to II and stabilized with an intramolecular hydrogen bond formed between one of the amino group hydrogen atoms and the π-electrons of the double bond, whereas two other forms correspond to I and they do not have this interaction. The physical properties of 4-isocyano-1-butene have been subject to only one investigation in the past by Chrostowska et al.,4 who performed a photoelectron spectroscopy study and density functional theory (DFT) calculations employing the CAM-B3LYP functional with the 6-311G(d,p) basis set. No conformational analysis was reported in this study. The wide variety of conformational properties of 4-substituted butenes and the fact that the microwave spectrum and the conformational properties of the title compound have not previously 1414
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Some of the theoretical results above warrant comments. The CCSD isocyanide group (N5C6) triple bond length is 116.6 pm in all conformers (Table 1). Fortunately, some equilibrium bond lengths of the isocyanide group exist. In HNC it is 116.83506(16) pm,29 and in CH3NC, a value of 116.9(1) pm has been determined.30 These bond lengths are in good agreement with the CCSD predictions (116.6 pm). The theoretical C3C4 bond lengths vary between 152.8 (conformers II and III) and 152.1 pm (IV and V) similar to the equilibrium CC bond length of ethane (152.2 pm).31 The theoretical C1C2 double bond lengths are in the 132.8− 133.0 pm interval (Table 1), whereas the equilibrium CC bond length of ethylene (H2CCH2) is 133.05(10) pm.32 There is an interesting variation in the C2C3C4 bond angle, which is 110.9° in I, and 5° larger in V, which may reflect steric repulsion in the latter conformer because of the close proximity of the ethylene part and the CH 2 NC moiety. The conformations of the five rotamers depend heavily on the C1C2C3C4 and C2C3C4N5 dihedral angles. The C1C2C3C4 dihedral angle is exactly 0° in IV and −0.3° in V, and a few degrees smaller than the “canonical” 120° in the remaining three forms. The C2C3C4N5 dihedral angle is exactly 180° in IV. In V, this angle deviates by 9.1° from 60°, possibly as a result of the said repulsion between the ethylene and CH2NC parts. The same dihedral angle deviates a few degrees from the characteristic ±60° and 180° in I−III. The CCSD electronic energy differences are shown in Table 2. It is seen that III is predicted to be the minimum-energy conformer, followed by I (+0.27 kJ/mol), II (+1.22 kJ/mol), IV (+2.09 kJ/mol), and V (+4.06 kJ/mol). No zero-point corrections are available at this level of theory, as noted above. The MP2 internal energy difference corrected for zero-point vibrational frequencies derived from entries in the SI, Tables 1S−5S, were 0, +0.32, +1.11, +3.64, and +4.13 kJ/mol for III, II, I, IV, and V, respectively. Both theoretical methods predict that III is the global minimum, but the increasing energy order of MP2 (III, II, I, IV, V) differs from that of CCSD (III, I, II, IV, V). The energy span of the five conformers of about 4 kJ/ mol is also similar. Microwave Spectrum and Pile-Up Series. The quantum chemical calculations above indicate that there are comparatively small energy differences between the five conformers of Figure 1. Survey spectra revealed a comparatively weak and extremely rich MW spectrum, which is compatible with the presence of several conformers separated by relatively small energy differences, just as predicted in the theoretical calculations above. Another factor that leads to low spectral intensity is the fact that each of these rotamers has several lowfrequency vibrational modes. In fact, the MP2 calculations (SI, Tables S1−S5) indicate that there are 6−7 fundamentals with frequencies below 500 cm−1 for each of the five conformers, three of which are typically lower than 200 cm−1. The two lowest vibrational modes can best be described as torsional vibrations about the C2−C3 and C3−C4 single bonds. Each of these several vibrationally excited states will consequently have a significant Boltzmann population and an associated MW spectrum resulting in a very crowded MW spectrum and overall reduction of intensity in agreement with observations. The survey spectra also revealed two series of characteristic and very rich pile-up series protruding from the weaker spectral background. Such pile-up series are characteristic for highly prolate rotors (Ray’s asymmetry parameter33 κ approaching −1) with a large μa and involve a-type R-branch transitions. A
intensities using small portions of dry ice to cool the waveguide. Selected measurements were also performed at room temperature. The pressure in the spectrometer cell was approximately 5−10 Pa during the measurements. The spectrum was recorded in the whole 35−80 GHz frequency interval. Selected measurements were also performed in other frequency regions. RFMWDR experiments20 were performed to unambiguously assign particular transitions.
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RESULTS AND DISCUSSION Quantum Chemical Methods. Several quantum chemical methods were used in the present calculations, which were performed using the Abel cluster of the University of Oslo. Second order Møller−Plesset perturbation calculations (MP2)22 were undertaken employing the Gaussian 0923 program package. Very high level ab initio coupled cluster calculations with singlet and doublet excitation, CCSD,24 were performed using the Molpro25 suite of programs. Peterson and Dunning’s26 correlation-consistent cc-pVTZ and cc-pVQZ basis sets, which are of triple- and quadruple-ζ quality, respectively, were used. Theoretical Calculations. Optimized structures, dipole moments, vibrational frequencies, and Watson27 quartic centrifugal distortion constants were first calculated at the MP2/cc-pVTZ level of theory for the five conformers I−V (Figure 1). The precautions of McKean et al.28 were observed when computing the centrifugal distortion constants. The results are shown in the Supporting Information (SI) Tables 1S−5S. Selected MP2 results are repeated in Table 2. The structures of I−V were finally optimized at the CCSD/ cc-pVQZ level using the MP2 structures as starting points. Calculations of CCSD vibrational frequencies, centrifugal distortion constants, and zero-point vibrational corrections are beyond our available computational resources. The CCSD principal inertial axes coordinates of the five conformers are listed in Table 6S of the SI together with their full geometrical structures. The structural parameters not involving hydrogen atoms are repeated in Table 1. The CCSD electronic energy differences and the principal axes dipole moment components are included in Table 2. Table 1. CCSD/cc-pVQZ Structuresa of Five Conformers of CH2CHCH2CH2NC Conformer: C1−C2 C2−C3 C3−C4 C4−N5 N5−C6 C1−C2−C3 C2−C3−C4 C3−C4−N5 C4−N5−C6 C1−C2−C3−C4 C2−C3−C4−N5
I
II
III
Bond Length (pm) 132.9 132.8 132.9 149.9 149.8 149.9 152.7 152.8 152.8 142.6 142.5 142.7 116.6 116.6 116.6 Angles (deg) 124.3 124.2 124.0 110.9 112.7 112.8 111.2 111.4 111.4 178.6b 178.3b 178.6b Dihedral Angle (deg) 115.6 115.8 118.4 177.2 −65.4 62.5
IV
V
133.0 150.0 152.1 142.6 116.6
132.9 150.2 152.1 142.7 116.6
126.5 113.9 110.6 178.7b
126.6 115.8 112.3 180.2
0.0 180.0
−0.3 −69.1
a
Structural parameters involving hydrogen atoms have been omitted. Full structures of the five conformers are given in Table 6S of the Supporting Information. bBent toward C3. 1415
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Table 2. CCSD/cc-pVQZ and MP2/cc-pVTZ Parameters of Spectroscopic Interest of Five Conformersa of H2C CHCH2CH2NC Conformer: A B C DJ DJK DK d1 d2
I 17861.3 1502.9 1477.5 0.271 −12.9 423 −0.0198 −0.00244
μa μb μc μtot
12.4 4.23 0.41 13.1
ΔE
0.27
II
III
Rotational Constantsb,c (MHz) 6091.9 8600.1 2571.7 2039.7 1972.4 1778.9 Quartic Centrifugal Distortion Constantsd (kHz) 5.22 1.60 −20.9 −20.6 31.4 97.7 −2.02 −0.356 −0.133 −0.0154 Dipole Momentb,e (10−30 C m) 6.51 8.66 11.1 9.28 0.75 1.67 12.9 12.8 Energy Differenceb,f,g (kJ/mol) 1.22 0.00
IV
V
14995.0 1693.7 1550.7
6130.0 2707.3 2135.2
0.181 −0.582 28.2 −0.0226 −0.00158
2.38 −8.50 15.8 −0.772 −0.0698
12.7 1.39 0.00 12.8
8.58 9.21 3.76 13.1
2.09
4.06
a
Minima on the potential energy hypersurface. bCCSD results. cRotational constants have been calculated from the structures in Table 1S of the SI. d MP2 results. S-reduction.27 e1 debye = 3.33564 × 10−30 C m. fElectronic energy. gElectronic energy of rotamer III: −653742.03 kJ/mol.
(2.97 GHz) showed that this series undoubtedly had to be assigned to conformer I, while the weaker pile-up series (B + C ≈ 3.24 GHz) must belong to IV. Assignment of the Spectrum of Conformer I. Detailed description of the assignment of the spectrum of I is presented here, while that of rotamer IV is discussed below. While the identifications of the J quantum numbers of a aR-series are obvious, the assignments of the K−1 lines belonging to a particular pile-up are often problematic. Stark modulation patterns and RFMWDR experiments were very helpful in this respect. The MP2 centrifugal distortion constants were also found to be very useful to identify the correct K−1 quantum number. The lines were fitted to Watson’s Hamiltonian in the Sreduction form using the Ir-representation27 employing Sørensen’s program Rotfit.34 The assignments from the pileups produced accurate values for the B and C rotational constants and the DJ and DJK quartic centrifugal distortion constants. Rather inaccurate values of the A rotational constant were obtained from these transitions. The K−1 = 1 pair of lines, which are well separated from the pile-ups, is much more sensitive to the A rotational constant than the other aR-lines. Searches for these lines soon met with success and produced an A rotational constant that was accurate to within roughly ±20 MHz (one standard deviation). A μb of 4.23 × 10−30 C m is predicted for I (Table 2), and searches were made for the strongest b-type transitions. These lines, which are predicted to be much weaker than the aR-lines, were not found presumably because of their weakness and the crowded nature of the spectrum. No transitions displayed a resolved hyperfine structure due to quadrupole coupling of the 14N nucleus. This was expected because the quadrupole coupling constants of 14N nuclei are relatively small for isocyanides. The quadrupole coupling constant of the 14N nucleus of CH3NC is, for example, only 0.4894(4) MHz.35 The MP2 14N quadrupole coupling constants, which are indeed relatively small, are given at the end of Table 1S.
typical example taken from the strongest pile-ups series is shown in Figure 2. This figure demonstrates the characteristic
Figure 2. Portion of the MW spectrum taken at a field strength of about 110 V/cm. This spectral region is dominated by absorption lines mainly associated with the J = 23 ← 22 a-type transitions of I. Values of the K−1 pseudo quantum number is listed above several peaks belonging to the ground vibrational state. The two lines with K−1 quantum numbers 3 and 4 are not well resolved and well modulated. Most of the remaining unlabeled transitions belong to vibrationally excited states. The intensity (Y-axis) is in arbitrary units.
spectral richness of these pile-ups. Overlapping of spectral lines of the ground and vibrationally excited states occurs frequently, especially at the higher frequency end of the pile-up region. Consecutive pile-ups are separated by almost exactly the sum of the B and C rotational constants. It was found that B + C ≈ 2.97 GHz for the stronger series, and ≈3.23 GHz for the weaker series. There are two candidates, namely, rotamer I (κ = −0.99 and B + C ≈ 2.98 GHz (Table 2)) and conformer IV (κ = −0.92 and B + C ≈ 3.24 GHz (Table 2)), that have these properties. The sum of B + C of the strongest pile-up series 1416
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the frequencies of b-type transitions, which were readily assigned. The assignments were then gradually extended to include higher and higher values of the J quantum number. Ultimately, 266 transitions with Jmax = 45 and K−1max (Table 8S of the SI) were used to obtain the spectroscopic constants shown in Table 4.
The assignments were gradually extended to include transitions with J up to 29 and a maximum value of K−1 = 17. A total of 270 aR-transitions shown in Table 7S in the SI were used to derive the spectroscopic constants shown in Table 3. Four of the quartic centrifugal distortion constants DJ, DJK, Table 3. Spectroscopic Constantsa of the Ground Vibrational State of Conformer I of H2CCHCH2CH2NC A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HKJc (Hz) rmsd Ne
Table 4. Spectroscopic Constantsa of the Ground and First Vibrationally Excited States of Conformer III of H2C CHCH2CH2NC
17558(20) 1496.8632(57) 1470.5239(57) 0.27292(70) −13.407(11) 423.3b −0.0151(35) −0.0191(98) −0.194(41) 1.588 278
a
S-reduction Ir-representation.27 Uncertainties represent one standard deviation. The spectrum is listed in Tables 7S in the SI. bFixed. cSextic constants other than HKJ were preset at zero in the least-squares fit. d Root-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 least-squares fit, and P is the number of spectroscopic constants. eNumber of transitions.
vibrational state:
ground
first excited torsion
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJb (Hz) HKJ (Hz) HK (Hz) rmsc Nd
8536.286(18) 2034.3132(32) 1770.7575(32) 1.5778(94) −20.880(22) 150.21(70) −0.34524(13) −0.016291(51) 0.0358(87) 1.82(43) −56(12) 1.875 266
8478.289(30) 2042.1809(32) 1777.2100(31) 1.676(11) −21.072(60) 100.0(14) −0.36697(27) −0.01742(16) 0.023(11) 2.8(12) −35(22) 1.683 177
a
S-reduction Ir-representation.27 Uncertainties represent one standard deviation. The spectra are listed in Tables 8S and 9S in the Supporting Information. bSextic constants other than HJ, HKJ, and HK were preset at zero 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. dNumber of transitions.
d1, and d2 could be determined. The first two of these are quite accurate. DK depends to a very minor degree on the aRtransitions and no significant value was obtained for this centrifugal distortion constant. It was therefore preset at the MP2-value in the least-squares fit. One sextic centrifugal distortion constant, namely, HKJ, was also determined. The remaining sextic constants were preset at zero in the leastsquares fit. The experimental spectroscopic constants can now be compared with their CCSD and MP2 counterparts. The experimental B and C rotational constants (Table 3) deviate by less than 0.4% and 0.5%, respectively, from the CCSD values (Table 2), whereas A differs by about 1.7% from its CCSD counterpart. Difference of this order of magnitude is to be expected because the CCSD and experimental rotational constants are defined differently. The experimental constants are obtained from an effective structure, whereas the theoretical constants are calculated from an approximate equilibrium structure. However, the good agreement between the CCSD (Table 2) and effective rotational constants (Table 3) is an indication that the CCSD structure in Table 1 is accurate, which is not surprising given the high computational level it is based on. There is very satisfactory agreement between the MP2 and experimental quartic centrifugal distortion constants in the cases of DJ and DJK (better than 4%). The uncertainties of the experimental constants d1 and d2 are quite large. A comparison with their MP2 equivalents is therefore not warranted. Conformer III. Relatively strong a- and b-type spectra were expected for this form due to the fact that μa = 6.51 and μb = 11.1 × 10−30 C m (Table 2). Searches for the aR-spectrum soon met with success using the spectroscopic constants of Table 2 to predict the spectrum. The preliminary spectroscopic constants obtained from these transitions were used to predict
It is seen from this table that accurate values have been obtained for all five quartic centrifugal distortion constants. Three sextic constants HJ, HKJ, and HK were also obtained, but they have rather large standard deviations. Comparison of the rotational constants (Table 4) with their theoretical counterparts (Table 2) reveals relatively small deviations of 0.7%, 0.3%, and 0.5% in the cases of A, B, and C, respectively, whereas deviations of 1.2%, 1.4%, 7.1%, 3.2%, and 5.5% were found for DJ, DJK, DK, d1, and d2 for the centrifugal distortion constants, which is considered to be very satisfactory. The spectrum of one vibrationally excited state was also assigned in the same manner as the spectrum of the ground vibrational state. The spectrum, consisting of 177 transitions, is found in Table 9S, whereas the spectroscopic constants are listed in Table 4. Relative intensity measurements yielded 73 (25) cm−1 for this vibration, which is assumed to be the lowest torsional mode, compared to 78 cm−1 obtained in the MP2 calculations (Table 3S). Conformer IV. This very prolate rotor has a spectrum that in principle is very similar to that of I but significantly less intense, and it was assigned in the same manner as the assignment of the spectrum of I. Only aR-transitions with Jmax = 29 and K−1max = 11 were assigned. The spectrum is listed in Table 10S in the SI and the resulting spectroscopic constants are given in Table 5. The DK, d1, and d2 centrifugal distortion constants were held fixed at the MP2 values in the fit in this case. 1417
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Table 5. Spectroscopic Constantsa of the Ground Vibrational State of Conformer IV of H2C CHCH2CH2NC A (MHz) B (MHz) C (MHz) 2Pccb (10−20 u m2) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HKJd (Hz) rmse Nf
could be one reason why we were not able to assign its spectrum. Energy Differences. The internal energy differences between the three conformers were obtained from comparison of the intensities of several selected transitions of the ground states of the three rotamers using the procedure outlined by Esbitt and Wilson.36 The dipole moment must be known in order to derive the energy difference. Experimental dipole moments are not available. The CCSD dipole moments (Table 2) were therefore used because dipole moments at this level of theory are expected to be very close to their ground-state counterparts. Conformer I was found to be the global energy minimum. The results were EIII − EI = 1.1(7) kJ/mol and EIV − EI = 2.9(7) kJ/mol. The statistical weight of IV was taken to be 1, whereas the statistical weight of I and III was assumed to be 2. The quoted uncertainty of ±0.7 kJ/mol is one estimated standard deviation. The uncertainties of the dipole moment components have been taken into consideration in this estimate. These values differ a little from the CCSD results (Table 2), which predict III to be the global minimum with I 0.27 and IV 2.09 kJ/mol higher in energy. The CCSD energy differences are not corrected for zero-point vibrational effects. The lower-level MP2 calculations corrected for these effects predict I to be 1.11 and IV to be 3.64 kJ/mol higher in energy than III.
14861.5(52) 1684.1114(73) 1541.8417(61) 6.316(14) 0.1818(13) −0.536(43) 28.2c −0.0226c −0.0158c 0.77(37) 1.202 112
a
S-reduction Ir-representation.27 Uncertainties represent one standard deviation. The spectrum is listed in Tables 10S in the SI. bDefined by Pcc = (Ia + Ib − Ic)/2, where Ia, Ib, and Ic, are the principal moments of inertia. Conversion factor: 505379.05 × 10−20 MHz u m2. cFixed. d Sextic constants other than HKJ were preset at zero in the leastsquares fit. eRoot-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, and P is the number of spectroscopic constants used in the fit. fNumber of transitions.
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CONCLUSIONS CCSD/cc-pVQZ and MP2/cc-pVTZ calculations predict that 4-isocyano-1-butene has five conformers, denoted I−V. Rotamer III is computed to have the global minimum electronic energy and V represents the maximum with an energy that is 4.06 kJ/mol higher than that of III. The CCSD structures of each of these forms reveal rather small deviations from expected geometries, with the exception of V, where there is indication of intramolecular repulsive interaction because the C2−C3−C4−N5 dihedral angle opens up to −69.1° from the typical −60° bringing the isocyano and vinyl groups further apart. The comparatively weak MW spectra of three of these rotamers denoted I, III, and IV were assigned and spectroscopic constants were obtained for each of them. Rotamers II and V may coexist with the other three forms, but extensive searches for their spectra did not result in assignments. The experimental and CCSD rotational constants of I, III, and IV are in very good agreement in all cases, which is evidence that the CCSD structures are good estimates of the equilibrium structures of the conformers. The MP2 quartic centrifugal distortion constants are in good agreement with their experimental counterparts in all cases where meaningful comparison can be made. Relative intensity measurements indicate that I is the global minimum with an internal energy that is 1.1(7) kJ/mol less than the energy of III and 2.9(7) kJ/mol less than the energy of IV. The CCSD computations predict another energy order with III as the global minimum and I and IV with 0.27 and 2.09 kJ/ mol, respectively, higher energies.
Comparison of the experimental rotational constants and quartic centrifugal distortion constants (Table 5) and the CCSD and MP2 equivalents (Table 2) reveals similar, relatively small differences as found in the cases of I and III discussed above. Conformer IV has a symmetry plane and two sp3-hybridized carbon atoms, C3 and C4 (Figure 1), with four out-of-plane hydrogen atoms. The value of the planar moment, defined by Pcc = (Ia + Ib − Ic)/2, where Ia, Ib, and Ic, are the principal moments of inertia, depends only on these hydrogen atoms. Table 5 reports the value of 2Pcc to be 6.316(14) × 10−20 u m2. The CCSD value calculated from the structure in Table 1 is 6.19 (same magnitude and units). Interestingly, the corresponding value found for the antiperiplanar conformer of 4isocyano-1-butyne (HCCCH2CH2NC), which also has four similar out-of-plane hydrogen atoms, is −6.278903(65) × 10−20 u m2,10 close to that obtained for IV. Searches for the Spectra of II and V. The assignments presented above for the three rotameric forms comprise the majority of the strongest transitions of the spectrum. However, many relatively strong transitions remain unassigned, but it is assumed that most of them belong to the spectra of unassigned vibrationally excited states of the three conformers. Comprehensive searches for the spectra of II and V were undertaken using both Stark and RFMWDR spectroscopies using the spectroscopic constants of Table 2 to predict their spectra. We did not succeed in assigning the two spectra. It is likely that the theoretical spectroscopic constants of II and V are just as accurate as their counterparts of I, III, and IV. The fact that no assignments were achieved can indicate that they have somewhat higher energies than the assigned three forms making their spectra even weaker than the spectra of I, III, and IV. The high CCSD energy of V (4.06 kJ/mol; Table 2) points in this direction, whereas it is harder to understand why the spectrum of II was not found. The fact that this conformer would lack comparatively strong and characteristic aR-pile-ups
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ASSOCIATED CONTENT
* Supporting Information S
Results of the theoretical calculations, including energies; molecular structures; dipole moments; vibrational frequencies; rotational and centrifugal constants; and 14N nuclear quadrupole coupling 1418
electronic harmonic distortion constants.
dx.doi.org/10.1021/jp4122134 | J. Phys. Chem. A 2014, 118, 1413−1419
The Journal of Physical Chemistry A
Article
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Microwave spectra of the three conformers. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +47 2285 5674; Fax: +47 2285 5441; E-mail: harald.
[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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