Article pubs.acs.org/JPCA
Molecular Structure of Methyldifluoroisocyanato Silane: A Combined Microwave Spectral and Theoretical Study Gamil A. Guirgis,*,† Jason S. Overby,† Michael H. Palmer,‡ Rebecca A. Peebles,§ Sean A. Peebles,§ Lena F. Elmuti,§ Daniel A. Obenchain,§ Brooks H. Pate,∥ and Nathan A. Seifert∥ †
Chemistry and Biochemistry, College of Charleston, Charleston, South Carolina 29424, United States School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, United Kingdom § Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, Illinois 61920, United States ∥ Department of Chemistry, University of Virginia, McCormick Road, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States ‡
S Supporting Information *
ABSTRACT: The structure of methyldifluoroisocyanato silane, MeF2SiNCO (2), has been studied by molecular rotational spectroscopy. The rotational spectrum has a complicated structure from 14N nuclear quadrupole coupling and internal rotation of the methyl group. Cavity Fourier-transform microwave spectroscopy measurements were important for providing high spectral resolution to analyze the quadrupole and internal rotation fine structure. Broadband chirped-pulse Fourier-transform microwave spectroscopy was used to achieve high measurement sensitivity making it possible to observe the lower abundance C, N, O, and Si isotopologues in natural abundance for structure determination. Analysis of the microwave spectrum of the most abundant isotopomer of MeF2SiNCO (2) yields the rotational constants: A = 3827.347(7), B = 1264.5067(14), and C = 1240.6182(11) MHz. The spectrum has been analyzed in the Ir representation for Cs symmetry, with inclusion of the 3-fold rotor (V3 = 446(50) cm−1). A partial substitution structure was obtained for the C, Si, N, and O atoms. The analysis was assisted by calculations of the equilibrium structure, using a 6-311++G (3df, 3pd) basis set, with calculations at each of the B3LYP, MP2, and CCSD(T) levels. The calculated and experimental rotational constants are only consistent with a trans-orientation at each of the HCSiN, CSiNC, and SiNCO centers; there is relatively close agreement between the experimental and the theoretical structures, especially at the CCSD(T) level. In addition, the observed low value for the 14N quadrupole coupling term (χbb − χcc) implies a wide SiNC angle, which is consistent with the calculated values: 165.3° (B3LYP), 157.6° (MP2), and 157.4° (CCSD(T)). The skeletal bending potential is discussed.
1. INTRODUCTION
amplitude motions (LAM) in the molecular normal vibrational modes, are characterized by vibrationally excited states close to the ground state.2−6 Spectral analyses under these circumstances often show extreme centrifugal distortion;2−6 these situations complicate both MW and infrared (IR) spectral interpretation. Substantial differences between those structures derived from spectral methods (such as MW or IR) and the corresponding electron diffraction (ED) structures may result from the effects of vibrational averaging. These effects have been exhibited previously in several simple molecules, including methyl isothiocyanate,7 silyl isocyanate,7 and silyl isothiocyanate.8 A further complexity for substances showing LAM, is that high level theoretical calculations are necessary to give satisfactory agreement between theory and experimental structures. For the HF2SiNCO molecule, the CCSD(T) optimized structure (1, in Figure 1) has A, 7.1065; B, 1.5178;
Recently, we reported a combined microwave spectral (MW) and theoretical study of the HF2SiNCO molecule (CAS Registry Number: 83620−28−4; 1, Figure 1).1 This is an example of a quasi-linear or “floppy” molecule; the large
Received: March 15, 2012 Revised: July 2, 2012 Published: July 3, 2012
Figure 1. Equilibrium structure for the closely related compound HF2SiNCO at the CCSD(T) level. © 2012 American Chemical Society
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C, 1.3110 GHz, to be compared with the experimental MW values A, 7.1112; B, 1.5657; C, 1.3475 GHz, a satisfactory result. We now report the synthesis of MeF2SiNCO (2, Figure 2) not previously recorded in the literature; it is the methyl-
as detailed below, as well as by its MW spectrum; compound 2 is not indexed in Scifinder Scholar. The NMR chemical shifts for 2 (in CDCl3 solution, measured from Me4Si) for each nucleus were 1H δ 0.534, 13C δ −6.127, and 19F δ 130.184 ppm. The 1H and 13C signals show (triplet) coupling to 19F. The geminal two-bond (2J) 13C− Si−19F spin coupling observed in the 13C spectrum is 21.3 Hz, while that for the vicinal coupling H−C−Si-F (3J) is 4.58 Hz, observed on both H and F nuclei. The present 1H and 19F NMR results are consistent with those for related compounds, including MeSiHF2,11 which has (1H) τCH 9.68, where τCH = 10-δCH, and 19F δ 135 ppm, and vicinal coupling H−C−Si-F (3J) 4.17 ± 0.10 Hz.12 The MW absorption discussed below also confirms the identity of the compound.
Figure 2. Variables (excluding dihedral angles) used for the structural analysis in both the MW and the theoretical study.
3. MICROWAVE SPECTRAL METHODS The complexity of the MW spectrum necessitated the use of several techniques on several instruments. Initial scans to identify the a-type transitions were performed at Eastern Illinois University (EIU) using both a 480 MHz bandwidth chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer,13 and a resonant cavity Balle-Flygare Fouriertransform microwave spectrometer.14,15 The CP-FTMW spectrometer was used to carry out an initial (2000 gas pulses giving a total of 2000 free induction decays) search of selected regions between 7 and 18 GHz. Gas-phase samples were expanded through the (0.8 mm) orifice of a pulsed nozzle (General Valve Series 9) into the vacuum chamber; samples were prepared in a mixture of He/Ne (17.5:82.5%, BOC Gases) at a total pressure of about 2 atm, giving a concentration of methyldifluoroisocyanato silane (2) of approximately 0.05%. Nuclear quadrupole hyperfine structure arising from the 14N nucleus was further complicated by splittings of a similar magnitude arising from the methyl group internal rotation, leading to significant spectral congestion in some regions. Given the complexity of the spectrum, a full (6.5−18 GHz) broadband scan was then carried out at the University of Virginia (UVa) to ensure that no additional states were overlooked. The 6.5−18 GHz broadband spectrum was obtained using the 11.5 GHz bandwidth CP-FTMW spectrometer at UVa, described in detail previously.16 The gas sample was prepared by using approximately 0.1% of 2, with Ne (GTS Welco) as the carrier gas. The sample was expanded into the vacuum chamber through three pulsed nozzles (General Valve Series 9) with a backing pressure of about 1 atm. The MW spectrum was obtained from the Fourier-transform of the average of 400000 time-domain free induction decay signals. The strongest transition for 2 was observed at approximately 3000:1 signalto-noise. In addition to the target molecule, a strong spectrum of MeSiF2Cl was observed as an impurity in the sample. The sensitivity was high enough to assign the MW spectra of several heavy atom isotopologues (Si, C, N, and O) of MeF2SiNCO in natural abundance.
derivative of HF2SiNCO (1). By analogy with 11 and its dichloro-analogue (CAS Registry Number: 53007−23−1), we refer to 2 as methyldifluoroisocyanato silane. MeF2SiNCO (2) shows additional complexities when compared with HF2SiNCO (1), because in addition to bending of the SiNC angle and cis-/ trans-isomerism of the SiNCO group, internal rotation of the H3C-SiF2− group will also occur. The present approach to solving the structural and conformational issues for compound 2 combines an extensive MW experimental study with coupled cluster (CCSD(T)), MP2, and DFT (B3LYP) calculations of the equilibrium structure. Most previous equilibrium structures for azides, isocyanates and isothiocyanates (R-NXY, where XY is NN, CO, or CS, and R = CH3, SiH3, etc.) show that if the heavy atom skeleton is nonlinear, then both the CNXY and SiNXY dihedral angles have a trans orientation.9,10 However, for HF2SiNCO, the observed MW rotational constants, when taken in conjunction with reasonable values for the bond lengths and angles, only support the cis-SiNCO conformer as the detected species; no lines attributable to another conformer were found. Because the trans-conformer has a marginally larger calculated dipole moment, it appears that the greater abundance of the cisconformer must be an electronic effect. In the theoretical study of HF2SiNCO,1 all three methods used found the lowest energy conformer has trans-HSiNC and trans-SiNCO dihedral angles, although the energy difference is very small. The internal rotation barrier for the HF2Si group was less than 1 cm−1.
2. SYNTHETIC PROCEDURE FOR METHYLDIFLUOROISOCYANATO SILANE Methyldichlorosilyl isocyanate (3) was prepared (Figure 3) by the reaction of freshly distilled methyltrichlorosilane (4) with
Figure 3. Synthetic route for methyldifluoroisocyanato silane.
4. THEORETICAL METHODS The electronic structure for the title compound (2) was determined using the Gaussian 0917 and MOLPRO18 suites of programs; electric field gradients (EFG) and derived 14N nuclear quadrupole coupling constants (NQCC)19 were determined using GAMESS-UK.20 Several methods were utilized, including B3LYP, MP2, and CCSD(T). To gain
silver cyanate, at room temperature for 4 days. The volatile dichloro-compound (3) was separated from unchanged starting material by trap-to-trap distillation at −72 °C. The product (3) was fluorinated using freshly sublimed antimony trifluoride, in the absence of a solvent, at room temperature. The final product (2), also separated by trap-to-trap distillation, was identified by IR and nuclear magnetic resonance spectroscopy, 7823
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information concerning the molecular conformation, including Cs and several C1 symmetry conformers, a series of basis sets were used. The final results recorded here used a 6-311++G (3df, 3pd)21,22 basis set; this is the largest Pople-style basis available23,24 and contains H[4s3p1d] and C,N,O,F[1s4sp3d1f] (contracted) functions. These are triple-ζ valence with polarization, together with diffuse functions on all atoms. The total number of Cartesian basis functions (380), was reduced to 335 by the harmonic (5d, 7f) option. Closely related bases have been used satisfactorily in studies of silyl azide,25 silyl isothiocyanate,26 and, of course, HF2SiNCO.1
Table 1. Spectroscopic Parameters for the Methyldifluoroisocyanato Silane (Most Abundant Isotopologue) Compared with Calculated Values for the Conformer with HACSiN = CSiNC = SiNCO = 180.0° Cs symmetry parameter A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) Na Δνrmsb (kHz) Iaa(u Å2) Ibb (u Å2) Icc (u Å2) Paac(u Å2) Pbb (u Å2) Pcc (u Å2) χaad (MHz) χbb (MHz) χcc (MHz) χab (MHz) (χbb − χcc) (MHz) V3 (cm−1) F0f (GHz) Iαg (u Å2) θαah (deg)
5. RESULTS The principal MW results for methyldifluorisocyanato silane (2) are summarized in Table 1. The set of possible conformational isomers is shown in Supporting Information (Figure S1). The only possible conformer where there is comparability to the observed rotational constants is the one where each of the dihedral angles HCSiN, CSiNC, and SiNCO is 180°; the structural data for this conformer for each method are shown in Table 2, and the molecule in the inertial axis frame is in Figure 4. The conformation of the CH3SiF2 unit is “ethane”-like (Figure 2), leading to a dihedral angle HACSiN of 180°. 5.1. Methyl Group Rotational Spectrum. All of the transitions in the MW rotational spectrum were doubled owing to internal rotation splitting from the methyl group. For many transitions this A and E state rotational splitting was similar in order of magnitude to the nuclear quadrupole hyperfine structure arising from the 14N nucleus, and hence required special attention. The final analysis of the spectrum was carried out using the XIAM program;27 this program uses a combined axis method to perform a simultaneous fit for internal rotation and quadrupole hyperfine structure. A set of the observed transitions and their assignment for the normal isotopologue is shown in the Supporting Information (Table S1). Only a small subset of observed transitions could be precisely fitted using this approach, because many transitions (observed in the full broadband scan measured at UVa) showed unresolved A−E and/or quadrupole hyperfine splittings; however, even for lines that were not resolved well enough to obtain precise frequency measurements, it was clear that predictions based on the fitted transitions agreed very well with the observed spectrum. Many b-type A-state transitions were observed to have approximately equally spaced “satellite” lines, with a virtually identical nuclear quadrupole hyperfine structure, that were within a few MHz of the A-state transition. When the internal rotation fit was implemented, half of these lines were readily identified as E-state b-type transitions; the remaining “satellites” were ultimately assigned to E-state c-type transitions. These ctype transitions are dipole forbidden based on the expected abplane of symmetry in the molecule, but the similar magnitudes of the asymmetry splitting and A−E splitting in this near prolate top (κ = −0.98) cause mixing of K states due to torsional-rotation interactions, which ultimately causes c-type E-state transitions to become allowed.28 Confirmation that the observation of c-type selection rules is not due to a lack of Cs symmetry in the molecule comes from the absence of corresponding A-state lines for these transitions. It would be reasonable to observe rotational transitions in excited vibrational states of the low frequency modes of this quasi-linear molecule; however, no evidence of excited vibrational states was observed.
microwave spectrum 3827.347(7) 1264.5067(14) 1240.6182(11) 0.50(6) 65.4(3) −67.3(13) −0.07(3) 70 9.0 132.00442(2) 399.6650(5) 407.3607(4) 337.4907(5) 69.8699(5) 62.1743(5) 1.896(7) −0.920(17) −0.977(10) 0.056(12) 446(50)e 157(3) 3.22(7) 58.0(8)
B3LYP
MP2
CCSD(T)
3753.1 1232.6 1209.7 0.35 17.4 −13.2 0.063 −8.7
3768.9 1248.5 1225.2 0.42 15.6 −11.7 0.069 −17.6
3778.7 1248.8 1225.1
134.94 410.75 419.25 347.53 71.721 63.223 2.0198 −0.9973 −1.0225 0.1467 0.0252 433
134.19 406.07 413.26 342.57 70.693 63.499 0.6803 0.2180 −0.8983 0.7039 1.1163 440
133.75 404.71 412.55 341.75 70.796 62.950 2.0364 −0.9749 −1.0615 0.3437 0.0865
a N is the number of fitted hyperfine transitions (this corresponds to a total of 15 unique rotational transitions. bRoot mean square deviation; Δν2rms = [Σ(νobs − νcalc)2/N] cPlanar moments, Paa = 0.5(Ib + Ic − Ia) = Σimiai2, etc. dThe electric field gradients in the inertial axis system are converted into 14N NQCC in MHz by use of the 14N nuclear quadrupole moment, with value 0.0201 barn, and conversion factor 234.96 for atomic units to MHz.19 eThe standard deviation in the internal rotation barrier (V3) from the fitting procedure is 11 cm−1; however, a strong correlation between V3 and F0 (correlation coefficient = −0.992) along with observed variation in V3 upon fitting different combinations of parameters indicate that the reported uncertainty of 50 cm−1 is more realistic. fRotational constant of the internal rotor. There is a strong correlation between V3 and F0 (correlation coefficient = −0.992); however, inclusion of both parameters was necessary to achieve a satisfactory fit of the observed frequencies. gMoment of inertia of the internal rotor, derived from F0. h Angle between the internal rotation axis and the a-axis of the molecule.
Assigned frequencies and residuals for the normal isotopologue are shown in the Supporting Information (Table S1), while fitted internal rotation parameters are listed in Table 1. Assuming the internal rotation axis is coincident with the Si−C bond, θαa gives an angle between the bond and a-axis of 58.0(8)°, in approximate agreement with the ab initio CCSD(T) value of 56.5° and the r0 structure value of 57.6° (see below). Measured centrifugal distortion and quadrupole coupling constants are also in reasonable agreement with the ab initio results. The 3-fold rotation barrier from the B3LYP and MP2 methods is relatively close to the experiment (Table 1 and Figure 5 (MP2)). 7824
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Table 2. Structural and Energetic Data for MeF2SiNCO with the 6-311++G(3df,3pd) Basis Seta method
B3LYP
total energy (au) −697.56985 geometric parameters (Å and deg) HAC 1.090 HBC 1.090 CSi 1.834 SiN 1.692 NC 1.196 CO 1.164 FSi 1.592 HACSi 110.6 CSiN 112.7 SiNC 165.3 NCO 177.7 FSiN 108.0 FSiNC ±57.4 CSiNC 180.0 SiNCO 180.0 HB,CCSiN ±60.0 HB,CCSiF ±61.1 dipole moment (Debye) μa 0.975 μb 2.395 μtotal 2.586
rs
MP2
CCSD(T)
−696.04078
−696.40497
1.088 1.088 1.827 1.694 1.206 1.170 1.591 110.5 111.5 157.6 177.2 110.3 ±57.7 180.0 180.0 ±59.8 ±61.0
1.092 1.092 1.831 1.693 1.205 1.166 1.586 110.8 111.5 157.4 177.2 108.4 ±57.6 180.0 180.0 ±59.8 ±61.1
1.030 2.459 2.666
1.549 2.007 2.536
1.849(1) 1.631(3) 1.231(2) 1.158(2)
110.8(2) 162(1) 179(24)
180(40)
r0
1.092b 1.092b 1.814(5) 1.667(21) 1.205(25) 1.152(5) 1.57646(8)c 110.8b 111.7(3) 159.8(9) 177.2b 108.4b ±57.6b 180.0b 180.0b ±59.8b ±61.1b
a
HACSiN = 180°. Experimental structural data are included for comparison. bFixed at value from CCSD(T) structure. cDistance determined directly from Pcc values (of all isotopes) so uncertainty is much lower than for in-plane atoms which are determined from least-squares fit of Ia and Ib of all isotopes.
Figure 5. Variation in energy for internal rotation of the methyl group at equilibrium using the MP2 method.
Figure 4. MeF2SiNCO equilibrium structure determined from the CCSD(T) study in the inertial axis frame. The close proximity of the Si and O atoms to the a-axis is important.
5.2. Substitution Structure. As a result of satisfactory intensity of transitions of several isotopologues of 2 in the broadband spectrum, an experimental rs substitution structure of the planar C−Si−N−C−O backbone was obtained using Kraitchman’s equations29−31 for the normal species and six isotopologues, corresponding to single substitutions with 29Si (4.67% abundance), 30Si (3.1%), 13C (1.1%), 15N (0.4%), and 18 O (0.2%). Kraitchman’s equations provide the absolute values of the atom coordinates in the principal axis system, and the theoretical structure is used to determine correct signs. All of the isotopologue spectra display similar patterns to the normal species, with resolvable A−E as well as nuclear quadrupole hyperfine splittings (except for 15N). Figure 6
shows a 120 MHz section of the broadband microwave spectrum, as well as the predicted transitions of 2 and its constituent isotopologues. A summary of the isotopologue fits, including rotational constants, can be found in Table 3, and complete assignment lists with residuals can be found in Supporting Information (Tables S2−S7). The isotopologue fits were performed with the V3 barrier height both fixed to the normal species value and as a fitted parameter. The fitted values for V3 (Table 4) were within the experimental error of the normal species 3-fold barrier height, providing some confidence in the validity of the isotopologue assignments, which typically fitted many fewer transitions than the normal species. 7825
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Table 4. Comparison of Fit Values for the Torsional Potential Barrier Height V3 for Each Isotopologue of MeF2SiNCOa parent C-1 13 C-2 29 Si 30 Si 13
V3 (GHz)
V3 (cm−1)
13383(329) 13637(157) 13423(327) 13673(157) 13366(154)
446(11) 455(5) 448(11) 456(6) 446(6)
a15
N and 18O were omitted due to lack of a sufficient number of assigned E symmetry state transitions in the fit.
all three methods employed. However, the most rigorous method, CCSD(T), shows the smallest b-component, together with the largest a-component. The moments of inertia and planar moments (Paa = 0.5 (Ibb + Icc − Iaa) etc., in cyclic permutation) are similar from all three methods (Table 1). Thus, the six values for Pij and Iij for the B3LYP series (y (u Å2)) in relation to the MW values (x (u Å2)) can be correlated using y = a + b*x; for example, the B3LYP series yields a = −0.69(32) and b = 1.031(1) (Supporting Information, Figure S2) where the standard errors are given in parentheses. The CCSD(T) method yields the overall best result, but the B3LYP results are given here as an example. However, a more obvious feature is that all calculated values from the three methods are numerically larger than the corresponding MW values (b > 1). The sums of the differences give mean errors, −3.0 (CCSD(T)), −3.6 (MP2), and −6.5 u Å2 (B3LYP) and show that the MP2 values are much closer to the CCSD(T) best result than is the DFT value. In contrast, while all three methods give relatively close values for the 14N nuclear quadrupole coupling constant χaa, the b- and c-axis values and their difference (χbb − χcc) are much better determined by the CCSD(T) and B3LYP methods than by the MP2 calculation. 5.3.2. Structural Results in Relation to the Energy Surface for Methyl Rotation and SiNC Bending. An initial objective of the theoretical study was to determine the lowest energy conformation (global minimum) for the molecule, and specifically whether it has C 1 or C s symmetry. Test computations with a variety of HACSiN, CSiNC, and SiNCO dihedral angles for reasonable bond and angle parameters showed that the most important single parameter for the MeF2SiNCO molecule is the angle SiNC. Of the eight Cs conformers that can be generated by systematic variation of the above-mentioned dihedral angles (Supporting Information, Figure S1), the only orientation consistent with observed rotational constants had all of these dihedral angles in the trans conformation. None of the C1 distortions introduced into the eight original Cs conformers appeared to lead toward a lower
Figure 6. Region of the broadband spectrum from 7440−7560 MHz. The normal species (red, bottom panel) 212−101 transition is approximately 3000:1 signal-to-noise with respect to its strongest hyperfine component. The top left panel shows the 313−212 transition of 4 of the isotopologues of MeF2SiNCO: 29Si (blue), 30Si (magenta), 13 C-1 (methyl, green), and the other 13C-2 (isocyanato, dark red). The top right panel shows the 15N 303−202 transition next to the equivalent 30 Si transition. The 18O isotopologue is not shown.
The Kraitchman method can be unreliable for precise calculation of atom coordinates when they lie close to the principal axes, owing to the variation of zero-point vibrational contributions to the moments of inertia upon isotopic substitution. This can lead to imaginary coordinates in the analysis. When this occurs, these coordinate positions are fixed to zero, generating some error in the rs parameters. In the rs structure for (2), the silicon and oxygen b coordinates were set to zero as a result of this effect (Table 5). An r0 fit was also performed;30−32 with the exception of the NCO angle, which was fixed to the CCSD(T) value, the positions of heavy atoms in the symmetry plane were fitted to Ia and Ib from all isotopologues, and the Si−F distances were fitted to the Pcc values from all isotopologues. This avoids the issues from the rs fit of inaccurate atom coordinates near the principal axes. The rs and r0 structures are compared with the CCSD(T) ab initio structure in Figure 7 and Table 2. Additionally, a comparison of the raw Kraitchman coordinates with the r0 method and with ab initio results can be found in Table 5. 5.3. Theoretical Results. 5.3.1. Direct Comparisons between Theory and Experiment. An effect observed on all three instruments used in measuring the rotational spectrum was that many of the observed a- and b-type relative intensities are approximately the same. This contrasts with the calculated dipole moments (Table 2), where the b-component is larger for
Table 3. Rotational Constants for the Six Assigned Isotopologuesa 13
C-1
A (MHz) B (MHz) C (MHz) N Δνrms (kHz)
3757.981(10) 1255.1750(18) 1224.3390(19) 17 26.9
13
C-2
3826.632(18) 1251.1370(17) 1227.6773(21) 20 24.9
29
Si
3827.3801(58) 1262.7308(11) 1238.9109(8) 40 20.4
30
Si
3827.4360(80) 1260.9920(10) 1237.2308(13) 38 17.7
15
N
3823.619(21) 1262.1399(27) 1238.1686(23) 13 21.5
18
O
3827.643(41) 1202.9838(87) 1181.4424(68) 8 27.3
Distortion and internal rotation parameters were held fixed to the normal species values. N and Δνrms are as defined in Table 1. C-1 is the carbon atom on the CH3 group and C-2 is the carbon atom on the NCO group. a
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Table 5. Coordinates of the MeF2SiNCO Backbone as Derived from the Kraitchman Substitution and Inertial Fit Methods, Compared with the Optimized Coordinates Calculated Ab Initioa Ab initio Substitution coordinatesb |a| C-1 C-2 O N 29 Si 30 Si
1.7211(9) 2.0713(7) 3.2184(5) 0.849(2) 0.753(2) 0.754(2)
c
r0 coordinates
CCSD(T)/6-311++G(3df,3pd)
|b|
a
b
a
b
1.575(1) 0.156(10) [0.000]d 0.306(5) [0.000] [0.000]
−1.7215 2.0765 3.2206 0.8885 −0.7496
−1.5660 −0.1441 −0.0094 −0.3431 −0.0338
−1.8345 2.0768 3.2341 0.8939 −0.7579
−1.5237 −0.1857 0.00497 −0.4546 −0.0405
a All values in Å. Kraitchman coordinates were obtained assuming a planar backbone. bPrincipal inertial axes denoted as a, b, c. cErrors in parentheses in units of the last digit, calculated using Costain’s estimates: δz = K/|z| (K = 0.0015 Å).34 dValues in square brackets constrained to zero.
Figure 8. MeF2SiNCO MP2 energy surface for SiNC angle bending; the B3LYP surface is very similar. The lowest energy point, determined for an SiNC angle of 160°, is marked. See text for further discussion of equilibrium SiNC angles at MP2, B3LYP, and CCSD(T) levels.
the B3LYP methods, and the former is shown in Figure 8. This surface is very different from parabolic, and numerical fitting is discussed below; the shallow global minimum shows the (interpolated) equilibrium SiNC angles are 165.3° (B3LYP) or 157.6° (MP2). Although we did not calculate the full potential energy surface, the equilibrium structure at the CCSD(T) level was obtained, giving a corresponding equilibrium angle of 157.4°. Differences for this SiNC parameter between the methods used here are not unusual, and such variations have been discussed previously.1 Indeed, the calculated SiNC angles for the cis-HSiNC conformer of HF2SiNCO (1) are 171.2° (B3LYP), 167.7° (MP2), 154.9° (MP4), and 154.6° (CCSD(T)).1 Numerical fitting of potential energy surfaces, especially where the minimum is very shallow, is normally given in terms of cm−1 for energy (y) and radian (θ) for angle (x). When the potential arises from a symmetrical environment, then we obtain a power series in x2, that is, even powers. The MeSiF2 group has an unsymmetrical environment for SiNC bending, and hence, the potential energy surface for bending of the SiNC angle allows both odd and even terms. In Figure 8 we show the energy surface for SiNC angle bending (expressed as both radians and degrees); the energy origin is chosen as the lowest energy point. Numerical fits of the energies (y) are given below, but an expanded portion near the central SiNC angle range is
Figure 7. Comparison of the MW derived structures (rs or r0; top and bottom, respectively) with the CCSD(T) equilibrium structure (large circles). The small circles represent the experimentally calculated coordinates; in the rs figure, the two overlapping small circles represent coordinates derived from the 29Si (salmon) and 30Si (lime green) isotopologues.
energy structure. Subsequently, it became apparent that the alltrans conformer was the only one having optimized rotational constants comparable with the microwave results. The methyl group internal rotation study, at both the B3LYP and MP2 levels, fixed the HACSiN dihedral angle (θ) at various angles between 0 and 180°, while allowing the remainder of the skeleton to relax. This led to the 180° value being established for the global energy minimum (Figure 5). The barrier to rotation at the eclipsed position, where θ = 0.0°, is 433 (B3LYP) and 440 cm−1 (MP2), respectively (Table 1). These are clearly very close to the value for the V3 potential of 446(50) cm−1 (Table 1). In the skeletal bending study, the SiNC angle was held at fixed values between 80 and 270°, while the remainder of the molecule was allowed to relax, but retaining Cs symmetry; the resulting energy surface is very similar for both the MP2 and 7827
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usual for physically acceptable “fits” of both experimental and theoretical data to polynomials. The results are similar to those for HF2SiNCO.1
shown in Figure 9, where the angle corresponding to the global energy minimum (x0) has been chosen as the origin. The theoretical “fits” are discussed below.
6. CONCLUSIONS Methyldifluoroisocyanato silane, MeF2SiNCO (2), presents some interesting structural features. The observed barrier to internal rotation of the methyl group (V3 = 446(50) cm−1) is considerably smaller than methyl group barriers determined for a variety of silanes (545−620 cm−1, see Table 7 of ref 33 for a summary). The very low value observed for (χbb − χcc) implies a nearly linear SiNC structure, in good agreement with the theoretical data. The agreement between the microwave spectral study and the theoretical study is sufficiently close to allow the assignment of the conformer as the Cs symmetry structure in Figure 2, where all three dihedral angles HACSiN, CSiNC, and SiNCO are trans. Superposition of the MW derived structures upon the CCSD(T) equilibrium structure (Figure 7, with small circles for the rs or r0 structure atoms) shows very satisfactory agreement (particularly considering that the MW values represent a structure that includes zero-point vibrational averaging, while the theoretical data provides an equilibrium structure). There is generally close agreement between the three theoretical methods used, and all of these give a satisfactory account of the 14N quadrupole coupling observed; however, none of the methods is superior in all cases where comparisons with experiment can be made.
Figure 9. MeF2SiNCO MP2 energy differences for SiNC angle bending near global minimum (x0 = 157.6° for this MP2 Figure) with polynomial fits (n) for (x − x0). The 21 data points are fit for n = 6 (blue), 8 (red), 9 (cyan), and 10 (black).
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In Figure 9 we express the SiNC angle scale as a difference (x − x0) from equilibrium; in the MP2 case shown, x0 is 157.6°. The curves shown are best “fit” polynomials to the full set of 21 data points, where the polynomial order is 6 (blue), 8 (red), 9 (cyan), and 10 (black). Although the energy differences are very small, a point of inflection apparently occurs above the global minimum, near to (x − x0) = 0.6 radians, giving an SiNC angle near 190°; the corresponding value for the B3LYP case is 194°; these give the complementary values 170 and 166° for the actual SiNC angle, respectively. The black best “fit” curve to the MP2 data even shows an apparent second minimum near to (x − x0) = 0.61 radians; this yields an apparent SiNC angle of 193°, with the complementary SiNC angle 167°. The polynomial “fits” of the 21 values of MP2 energy data (Figure 8) show a slow decline in the magnitude of terms; if the series is constrained to the 11 points closest to the global minimum, then an almost exact fit occurs with: y (cm−1) = 0.008(25) + 44.6(17) θ1 + 441 (1) θ2 − 1400 (4) θ3 + 1175 (9) θ4 + 234 (32) θ5 − 471 (15) θ6 radians; the standard deviations of the fit terms are shown in parentheses, the residual sum of squares is 6.9 × 10−4 cm−1, correlation coefficient (R2) is 1.000. None of the higher power fits in Figure 9 are as close to the level of “fit” afforded by the sixth order polynomial above, and it seems probable that the possible second conformer implied by the black curve in Figure 9 arises from higher polynomials giving too much emphasis on points further away from the minimum energy range. However, these features of the potential energy surface have very small energy differences, and zero-point vibrational energy will place the ground state well above these shallow features, making it impossible to observe distinct geometries. Most experimental “fits”1 do not go above quadratic and quartic. Both the declining range in polynomial term amplitudes and alternating signs in the sextic equation above (where only relative signs between terms are important) are
ASSOCIATED CONTENT
S Supporting Information *
Full refs 17 and 18, tables of measured rotational transition frequencies (Tables S1−S7) for all isotopologues, and two figures (Figures S1 and S2) showing possible conformers of MeF2SiNCO and correlation of B3LYP and MP2 results. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS Work at EIU and UVa was supported by the National Science Foundation (EIU, NSF RUI Grant CHE-0809387; UVa, NSF MRI-R2 Grant CHE-0960074).
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