Molecular Structure of Cyclopropyl (Isocyanato) Silane: A Combined

Subscriber access provided by UNIV LAVAL

Article

The Molecular Structure of Cyclopropyl (Isocyanato)silane: A Combined Microwave Spectral and Theoretical Study Sahand M. Askarian, Gamil A. Guirgis, Tamia Morris, Michael Henry Palmer, Brooks H. Pate, and Nathan A. Seifert J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10154 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

The Journal of Physical Chemistry

1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Molecular Structure of Cyclopropyl (Isocyanato) silane: A Combined Microwave Spectral and Theoretical Study Gamil A. Guirgisa, Sahand M. Askariana, Tamia Morrisa, Michael H. Palmerb, Brooks H. Patec, Nathan A. Seifertc a

Chemistry & Biochemistry, College of Charleston, Charleston, SC 29424 USA. School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3FJ, UK. c Department of Chemistry, University of Virginia, McCormick Road, P.O. Box 400319, Charlottesville, VA22904-4319, USA. b

Abstract The molecular equilibrium structures of two conformers (cis and gauche) of C3H5-SiH2NCO, have been deduced by a combination of microwave (MW) spectra at natural abundance including data from

13

C and

29,30

Si isotopomers, and ab initio calculations. The MW

rotational constants (RC) for the most abundant isotopes are cis: A = 4216.3617(64), B = 1225.76654(91) and C = 1037.31468(77) MHz, and gauche: A = 4955.55(79), B = 1094.9276(81) and C = 942.7031(80) MHz. The symmetric quartic centrifugal distortion constants have been evaluated for the cis conformer, using the Ir representation for CS symmetry. Only partial substitution structures (PSS) could be derived from the spectra after inclusion of the above isotopic combinations at each centre.

Using the PSS, the full

structures were determined by ab initio calculation of the equilibrium structures using coupled cluster singles and doubles with selected triples configuration calculations (CCSD (T)); the two conformers have an energy difference of 228 cm-1 (cis lower than gauche). The similarity of the calculated and MW RC results confirm the identities of the two compounds. The more interesting cis-conformer has bond lengths: C2-Si3, 1.9072(73), C2-C9 1.464(22) and C9-C10 1.4944(33) Å, and angles Si3-C2-C10 119.4(12)° and C9-C2-C10 57.1(12)°, with similar results for the gauche conformer. The Si3N4C5 angle is wide in the cis conformer (145º) and nearly linear in the gauche conformer (179º). New physical insights into the bonding of cis conformers of this type, have led the identification of an attractive force between the relatively crowded cyclopropyl and isocyanato groups in the cis-conformation. This is demonstrated by three methods: comparing electronic charges (both AIMALL and Mulliken analyses) in the pair of conformers show a relative shift of density between these groups in the cis compound. Comparison of the highest occupied MOs (HOMOs), show major mixing of density, exemplified by HOMO-1 in these structural units for the cis conformer, but which is absent for the gauche conformer. Finally,

ACS Paragon Plus Environment


Page 2 of 21

2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the nearly linear isocyanate moiety (and the molecular dipole moment) of the cis conformer point closely towards the connected C atom of the cyclopropyl ring, while the gauche conformer dipole moment is significantly different in direction and points towards the midpoint of the C2Si3 bond. Both the HCSiN torsional and Si-N=C bending surfaces connecting these conformers were explored at the Møller-Plesset second order perturbation theory level (MP2), which led to the exclusion of other conformers. The bending surface shows a very high amount of quartic potential function. 1. Introduction We recently reported combined microwave spectral (MW) and theoretical studies of three new silyl isocyanates, HF2SiNCO(1)1, Me-SiF2-NCO(2)2 and Me-SiHF-NCO(3)3 all of which showed wide SiNC angles. Synthesis of cyclopropyl (isocyanato)silane (C3H5-SiH2-NCO,4) and microwave spectral (MW) study showed the presence of two conformers (4a, cis and 4b, gauche) from the rotational constants (RC). Only the coordinates of the Si and C atoms for both conformers were obtained after inclusion of the

29,30

Si and

13

C isotopomer MW data.

The full molecular structures were determined by ab initio calculations, which showed that internal rotation of the silyl isocyanato and cyclopropyl groups leads to interconversion of these with a low barrier. The experimental presence of the more congested cis conformer, suggests attraction between the NCO and C3H5 units, and this led to a more detailed theoretical analysis than the previous studies of three related cyclopropylsilanes (C3H5SiH2X, with X = H, CN, CF3), which also exist as cis and gauche conformers.4,5,6 2. Synthesis and spectral details for the cis and gauche cyclopropyl (isocyanato) silanes (4a,b). Reduction of cyclopropyl trimethoxysilane (5) with lithium aluminium hydride gave cyclopropylsilane (6) (Figure 1), which gave bromocyclopropyl silane (7)7,8 on reaction with boron tribromide. After addition to dry silver cyanate at liquid nitrogen temperature, and slow warming to 40° over 15 hours, volatile by-products were removed under vacuum at -80ºC.


Page 3 of 21


3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Bulb-to-bulb vacuum separation using three sequential U-tubes cooled to -40, -60, and -196ºC gave the product cyclopropyl (isocyanato) silane (4a,b), which was further purified by trapto-trap distillation. Identification was by infrared and nuclear magnetic resonance (1H,

13

C,

and 29Si).

The NMR chemical shifts for each nucleus in cyclo-C3H5-SiH2-N=C=O (4a,b) (measured from Me4Si in CDCl3 solution) were:(a) 1H: H1 δ -0.10 ppm (as triplet of triplets), H2,3 δ 3.45ppm; (b) 13C: C1 δ -7.62, C2,3 -2.33, and CNCO 124.48 ppm, respectively; (c) 29Si δ -48.70 ppm. These 1H NMR coupling constants (Figure 1) are consistent with values for geminal, cis and trans couplings in related cyclopropane derivatives. The MW absorption discussed below also confirms the identity of the compound.



4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. The synthetic path to the pair of conformers of cyclopropyl (isocyanato) silane (4a,b), and the observed 1H NMR coupling constants using local numbering, which probably represents a time average over the two conformers. 3. The microwave study The rotational spectrum of sample (4a,b)

was studied using a chirped-pulse Fourier

transform microwave (CP-FTMW) spectrometer,9,10 operating in the 6.5−18 GHz range, at the University of Virginia. Most aspects of the methodology used in this study have been described previously,1-3 so only specific details of the present experiment are given here. The vapour of C3H5-SiH2-NCO was mixed with Ne gas (GTS Welco) to a total pressure of ca. 3.4 atm, containing the sample concentration at 0.2%. Using five pulsed nozzles and a repetition rate of 4 Hz, approximately 42,000 sample injection cycles were performed.


Page 4 of 21

Page 5 of 21


5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

During each cycle, 10 free induction decays (FIDs) of the supersonic expansion were collected. Each FID was recorded for 20 µs, and the resulting Fourier transform, consisting of approximately 2000 lines with signal to noise ratio greater than 3:1, has a dynamic range of 1800:1. The final time-domain spectrum consisted of a coherent average of 420,000 FIDs. The sensitivity of the experiment was high enough to detect the singly substituted 13C (1%), 29

Si (4.67%) and 30Si (3.1%) isotopologues in natural abundance for both conformers; these

assisted assignment of all C and Si atom coordinates in the two conformers, using Kraitchman’s equations.11 A summary of the results extracted is shown in Table 1. Further details of the MW study are given in the Supporting Information for Publication. Rotation Constants A / MHz B /MHz C /MHz (B-C) / MHz Derived bond lengths

Si3-C2 C2-C9 C2-C10 C9-C10 C2-C9-C10 C9-C2-C10 C9-C10-C2 Si3-C2-C9 H1C2Si3N4

Experimental Cis conformer Gauche conformer 4216.3617(64) 4995.55(79) 1225.76654(91) 1094.9276(81) 1037.31468(77) 942.7031(80) 188.4519 152.2245

Theoretical Cis conformer Gauche conformer 4557.16 5308.33 1160.93 1040.80 1009.89 915.68 151.04 125.12

Cis conformerb

Gauche conformer

Cis conformerb

Gauche conformer

1.9072(73) 1.464(22) 1.464(22) 1.4944(33) 61.5(12) 57.1(12) 61.5(12) 119.4(12) 179(3)

1.816(27) 1.545(29) 1.542(26) 1.461(17) 61.8(16) 56.5(13) 61.7(12) 117.8(21)

1.849 1.527 1.527 1.527 60.5 59.0 60.5 119.8 180.0

1.842 1.544 1.544 1.481 61.2

119.7 62.5

Table 1. Comparison of the principal microwave and theoretical results.a,b Footnotes to Table 1 a. Imaginary Kraitchman coordinates are set to zero. b. Methylene carbons assumed to be equivalent by symmetry 4. The theoretical study The electronic structures of 4a,b, including electric field gradients (EFGs) and derived

14

N

nuclear quadrupole coupling constants (NQCCs), were determined using the Gaussian-09 (G09)15 and MOLPRO16 suites; RHF and B3LYP single configuration calculations used G-09, while multi-configuration SCF, coupled cluster with singles, doubles and selected triples



Page 6 of 21

6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(CCSD(T) calculations used MOLPRO. We used the largest Pople-style basis (6-311G++ (3pd, 3df))[17] which was successful previously,1-3

for the equilibrium structures of

molecules 1-3. Subsequently, when conformational energy surface issues were studied at the lower level of MP2, we used the aug-cc-pVTZ basis set (H[4s3p2d], C,N,O,F[5s4p3d2f], Si[6s5p3d2f]),17,18 which allows additional flexibility in the wave-function.

The MP2

component of the correlation energy for the cis conformer represents -1.10547 a.u. (93%) of the total -1.19143 a.u. for the CCSD(T) calculations of the cis conformer, and justifies the MP2 approach in a reduced level study of the surface.

5. Structural Results and Discussion 5.1 Overall considerations. Since only the Si and all four C atom positions were fully derived from the MW study for each conformer, all atomic positions below are based on the ab initio studies; comparison of the partial experimental and complete theoretical molecular structures is deferred to the Supporting Information for Publication. The total energy difference (TE) of the cis and gauche conformers results from addition of several very large terms, namely nuclear-nuclear repulsion (N-N), electron-electron repulsion (E-E, 2-electron energy) and electron-nuclear attraction (N-E, 1-electron energy). These differ by several Hartree (1 a.u. = 27.211 eV = 219476 cm-1) between the conformers; in the present case the cis/gauche values are: N-N +333.4/+326.7; N-E -1449.5/-1436.1;

E-E +542.2/+535.5 a.u..

Because the cis conformer is more compact, both the N-N and E-E terms favour the gauche conformer. Only the N-E term favours the cis conformer, which represents an electron attraction between the atomic groups, part of which is between the C3H5 to NCO units, via the SiH2 unit. This same reasoning would apply to the earlier known cyclopropylsilanes C3H5SiH2X, where X = H, CN, CF3, which also exist as cis and gauche conformers,4-6 but this was not explored.


Page 7 of 21


7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The small resultant TE energy difference is best measured in wave-numbers for both the CCSD(T) and MP2 calculations. At the CCSD(T) level the cis is lower than the gauche by ~228 cm-1; at the MP2 level for the internal rotation study, cis is still lower but by an even smaller amount(~5.5 cm-1). Clearly the CCSD(T) relative energies are the most reliable for comparison with other species. However, it is the existence of the cis conformer which is the most interesting matter, and why crowded cis conformers of this type are occurring at all, and this is discussed below, in addition to the interconversion of the conformers. The partial equilibrium structures (Table 2) show that the N4C5 bond is longer than the C5O6 bonds for both conformers, which is similar to many previously noted studies;1-3 both dimensions are slightly smaller in the gauche conformer. The very wide SiNC angles found in our previous Papers1-3 occur here, but with considerable difference between conformers; the cis conformer has Si3N4C5 145.2º, whereas that for the gauche conformer is effectively linear (179.3º). This latter value is responsible for the high calculated A rotational constant which is very dependent upon the SiNC angle.1-3 Both of the conformers show the dihedral angle Si3N4C5O6 is 180º, but the bending of the N4C5O6 unit is small for both conformers.

H1 C 2 C2Si3 Si3N4 N4 C 5 C 5 O6 Si3H7 C2C9 C9C10 C2Si3N4 Si3C2C9 Si3N4C5 N4C5O6

Cis conformer Gauche conformer CS symmetry C1 symmetry Total energy / a.u. Total energy / a.u -575.10566 -575.10463 Bonds (Ẵ) and Angles (º) 1.086 1.076 1.849 1.842 1.744 1.729 1.209 1.179 1.175 1.156 1.479 1.469 1.527 1.544 1.527 1.481 105.8 110.4 119.8 119.7 145.2 179.3 176.1 179.9



Page 8 of 21

8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2. Selected bond lengths and angles from the equilibrium structures for both conformers. The full atomic numbering system is shown in the Figures below. 5.2 Conformational aspects. A more detailed theoretical study (Figure 3) of the full internal rotation the cyclopropyl group about the C2-Si3 axis was performed.

All geometric

parameters were optimized as a function of the dihedral angle H1C2Si3N4, using the MP2 method with an aug-cc-pVTZ basis set. Minima were obtained with a significant barrier at dihedral angles 60º and 300º (gauche) and 180º (cis), but much smaller energy difference (5.5cm-1 ), when compared with the CCSD(T) result.

Figure 3. The energy of the cyclopropyl (isocyanato) silane (4a,b) system as a function of the H1C2Si3N4 dihedral angle. This cis (180º) and gauche (60º) conformer energy difference using the aug-cc-pVTZ basis set with the more limited MP2 method is 5.5 cm-1. The lower interconversion barrier, with maxima at H1C2Si3N4 =120º and 250º is +10 cm-1 above the minimum. The barriers at 0 and 360º, are larger at ~30 cm-1, and are nearly 35 cm-1 above the global minimum. None of these energy values are corrected for zero point energy (ZPE) differences, which are expected to be small for a pair of conformers.


Page 9 of 21


9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5.3 Si3N4C5 bending studies. This energy surface was scanned in a similar manner, using an aug-cc-pVTZ basis set at the MP2 level; with all other geometric parameters were fully optimized at each SiNC angle; This angle data (x-x0) has been referred to the local maximum as zero radians (x0), as is usual for PE curves, while the energy axis is in cm-1. The results indicate (Figure 4) a very low energy SiNC angle bending double minimum well potential, as expected from our recent studies1-3 of the bending for other silyl isocyanates, where we used a similar procedure. The potential energy (PE) surface (Figure 4) is slightly unsymmetrical, ie it does not have left-right symmetry, owing to the unsymmetrical nature of the substituents; this allows ‘odd’ powers of the SiNC angle (x-x0) to be nonzero. In symmetrical cases, the quadratic and quartic terms must have different signs for a physical fit; that is largely true here, but small components of single power and cubic are present. The overall energy fit in relation to the SiNC angle is basically quartic, but contaminated by higher functions. Our best overall fit limited to quartic powers (Figure 4, in red), is not perfect (adjacent R2 0.998), since the two local minima lie on opposite sides of the fit curve. The normalised proportions (%) are (x-x0)4 80.3, (x-x0)2 11.7, (x-x0)3 7.3, (x-x0) 0.7%, showing the dominance of quartic terms.



10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. The energy of the cyclopropyl (isocyanato)silane (4a,b) gauche conformer energy as a function of the SiNC angle (black). The potential energy curve does not have left-right symmetry and there are two shallow minima. The group of very closely placed points near the origin SiNC (x-x0) = 0 shows the reproducibility of the data. 6. Electronic structure aspects which relate to the molecular structure. 6.1 Overall electron distribution. We used both Mulliken analysis and the AIMALL package,19 a recent and much expanded version of the earlier Atoms in Molecules (AIM) package (AIMPAC).20-22 In these electron distribution studies, using both Mulliken and AIMALL analyses, we used the same wave-functions containing all 60 electrons from all occupied MOs; this gives a direct comparison of these two methods for determining charges on atoms. AIM uses the concept of ‘atomic basins’, which are clearly delimited by zero electron density gradient surfaces between atoms, which define the atomic boundaries. In contrast, the Mulliken analyses, sums atomic populations of all basis functions about each atomic centre. These are fundamental differences, which can lead to confusion, and which we


Page 10 of 21

Page 11 of 21


11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

have discussed in our previous study.3 We show the equilibrium structures of both cis (Figure 5a) and gauche (Figure 5b) forms; superimposed are the directions of the dipole moments (DM, green), and the critical points (CP) where the zero gradient surfaces (‘atomic basins’) cross the internuclear regions (red), and Mulliken populations (blue). Both methods suggest that the conformers can be subdivided into three units, NCO, SiH2 and C3H5. The total populations as measures of electron densities (e) for these units, determined by conventional Mulliken populations (Figure 5a,b) are (cis/gauche): C3H5 -0.260/-0.196, NCO -0.436/-0.458 and SiH2 +0.636/+0.660e respectively; clearly the SiH2 groups are major electron donors. The cis C3H5 group is 0.064e more negative than the gauche conformer, indicating a small charge transfer overall from the NCO and SiH2 groups. Since only small changes are expected for a pair of conformers, the simplest interpretation of this energy differences is a net attraction between the C3H5 and NCO units, which overcomes the energy loss through folding in the congested cis conformer. Both the molecular dipole moments (DM) are nearly parallel to the almost linear NCO axis; the DM of the cis conformer (Figure 5a) lies close to the projection of the C5N4 bond axis, and points towards atom C2 of the cyclopropyl ring. In contrast, in the gauche conformer (Figure 5b), where the Si3N4C5O6 moiety is nearly linear, the DM lies towards the midpoint of the C2-Si3 bond axis. These directions are clearly dependent upon the Si3N4C5 angle, as well as the conformation of the C3H5 group, but the cis dipole moment again indicates a small overall flow of electrons towards the C3H5 ring. 6.2 The effective atomic radii and charge distribution. The AIMALL results show local ‘bond critical points (BCP)’ in each conformer (Figure 5a, 5b, in red) for the SiNCO structural units, CH and SiH bonds; these lie close to the interatomic axes as expected. For a bonded pair of atoms X-Y, this zero flux 2-dimensional surface is perpendicular to the bond,



12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and represents the ‘atomic basin’ or boundary of the pair of atoms. It follows that the closer that a BCP lies to one of the bond atoms, the smaller the basin, and the higher the charge on that atom; these follow the trends shown in our recent publication on molecule 3.3 However, some BCP of the C3H5 unit are clearly outside the internuclear C-C bonds, giving a ‘banana bond’ appearance,23 and this has been shown by appropriate projections for the conformers. Further, the C3H5 unit generates a further ‘ring critical point’ (RCP) relating to a density minimum within the ring atoms C2C9C10; in short, there is effectively a ‘hole’ in the ring.24 This is clearly seen for the gauche conformer in Figure 5b, where the conformation of the C3 ring lies close to the plane of the paper. In the cis conformer (Figure 5a), the RCP is projected onto the C2C9 bond as the left CP in Figure 5a; the right CP on the bond is the nearly equidistant pair of BCP.


Page 12 of 21

Page 13 of 21


13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The theoretical equilibrium structures of the cis and gauche conformers. The cis conformer dipole moment (green line) lies close to an extension of the OCN-axis towards the cyclopropane ring; this implies an attractive interaction between the N=C=O group and the ring, which is absent in the gauche case. Different projections are necessary to demonstrate the data for the separate conformers. 6.3 Electron density mixing in the occupied MOs. There are further indicators of the NCO and C3H5 interaction through study of the occupied MOs of the compound. Both isocyanates and cyclopropyl derivatives have characteristic low ionisation energy MOs.25,26 The two highest occupied MOs (HOMO and HOMO-1, Figure 6) show more delocalised density between the two structural units C3H5 and NCO, in the cis conformer than in the gauche conformer, where the orientations of the two conformers in the high density regions are similar. The gauche conformer shows most of the density in the NCO region for these two MOs, while the cis conformer shows a very mixed density especially in the HOMO-1. The set of 4 highest MOs are shown in the Supplementary Material.



14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cis conformer HOMO 30

Gauche conformer HOMO 30

Cis conformer HOMO-1 29

Gauche conformer HOMO-1 29

Figure 6. A comparison of the two highest occupied MOs for both conformers. The electron density contour is 0.02e for both series. Both the HOMO and HOMO-1 for the gauche compound show relatively highly localised density on the N=C=O unit; in contrast, these MOs in the cis case show rather more mixing and sharing of the population in both the C3H5 and NCO moieties.


Page 14 of 21

Page 15 of 21


15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7. Discussion and Conclusions The MW spectrum identified the presence in the gas phase, of two conformers of cyclopropyl (isocyanato)silane, with cis and gauche structures. Insufficient isotopic combinations were available to determine the full structure, but the atomic coordinates of two sets of

Si and

four C atoms, determined the individual conformations. The full structures, obtained by ab initio CCSD(T) calculations, confirmed the observed rotational constants for both conformers. At this level, the two conformers are close in energy (226 cm-1), while the less sophisticated MP2 level shows the same relative energy order (cis lower than gauche) with even smaller energy difference. The physical insight can be lost for small energy differences such as these, and we discuss this here; total energies are a small resultant from the summation of very large numbers. The nuclear (N-N) repulsion energies and 2-electron (E-E repulsion) energies both favour the gauche conformer, but this is outweighed by the electronnuclear (E-N) attraction energy which favours the more compact cis, and this must be a result of overall attraction of the isocyanato and cyclopropyl groups. Durig et al4 have reviewed the cis/gauche conformer stabilities for several related cyclopropanes. The present example with cis lower than gauche is similar to that for cyclopropyl (cyano) silane (123 ± 13 cm-1);4 the reverse order of stability occurs with cyanomethylcyclopropane, C3H5CH2CN, where the gauche is more stable,4 showing that even replacing SiH2 by CH2 can have a major difference. The halogenosilyl series is again inconsistent with the two larger halogen (X=Cl and Br) in C3H5SiH2X having a predominance of cis, while X=F leads to preference for gauche;4 this last result must indicate a repulsion between F and the C3H5 ring and lack of ring overlap with the smaller F-atom, while the less electronegative Cl and Br with their larger atomic radii leads to more a attractive interaction. In trying to develop these physical insights, we investigating the bonding of the cis and gauche conformers (4a,4b) by several methods. The electron density distributions were



16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

analysed by both Mulliken and AIM methods, using the same wave-functions in the two separate methods of density evaluation, and thus avoiding artefacts for each conformer. Although the overall charge distributions show similar polarities, the atomic populations show a small transfer of population from NCO to C3H5. The charges are larger in the AIM process, as a direct result of the atomic basin concept, where summation of density only occurs in a limited volume, determined by the critical points. Further insight into the interaction of these groups in the cis conformers of this type, is the observation of a very much more strongly delocalised nature of the cis HOMO-1 in particular, which contrasts with those of the gauche conformer, where the MOs density is more localised in the separate groups. The present approach can clearly be used much more widely, and in particular may well offer an explanation for some of the related cyclopropylsilanes C3H5SiH2X. In the case where X = CN, which also exists as a mixture of cis and gauche conformers,4-6 there is the possibility of overlap of the CN unit with the ring, but by a sideways approach as shown in Figure 3 of Durig et al.4 However, the similar behaviour of the cases where X = SiH3 or CF3 must involve other properties, since the polarities of these substituents are opposite. The similarity of these two groups lies in the local CH3 and CF3 group MOs of a1 symmetry which will project backwards towards the C3H5 ring across space in the cis conformer. The energy surface for both internal rotation around the H1C2Si3N4 torsion angle which defines the major difference between the two conformer structures, and also the bending of the Si3N4C5 angle have been studied. The latter shows an interesting unsymmetrical double minimum, a result of the C3H5SiH2 unit being non-symmetric; fitting of the data shows that there is a major quartic component in the bending mode, and is the highest that we have analysed so far.


Page 16 of 21

Page 17 of 21


17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Finally, the H-Si-H angles in both conformers are little different (cis 109º, gauche 110º) from the normal tetrahedral angle, even though two large substituents are present; this is an example of Bent’s Rule, originally expressed as "Atomic s-character concentrates in orbitals directed toward electropositive substituents".25 The major bonding component in the current molecules is via p- rather than s-orbital density on silicon. Acknowledgements GAG gratefully acknowledges the support of this study by the Camille and Henry Dreyfus Foundation by grant no. SI-14-007 and the Undergraduate Research and creative Activities at the College of Charleston. The NMR spectrometer at the College of Charleston is supported by the National Science Foundation under Grant No. 1429308. Supporting Information for Publication Contents of Supporting Information for Publication. S1. Further details of the MW study S2. Comparison of the experimental results with the complete theoretical data. S3. Basis sets and relative energies. S4. Numerical fitting of the SiNC bending surface. Table S1a. Rotational constants and centrifugal distortion constants for the cis conformer Table S1b Rotational constants and centrifugal distortion constants for the gauche conformer Table S2 Coordinates (rs) of the abundant isotopes derived from the MW study for the gauche-conformer in the inertial axis (a,b,c-system); derived bond lengths and angles. The numbering used here relates solely to the local unit, and not to the overall molecule. Table S3. Coordinates (Å) of the equilibrium structure of the cis-conformer. Table S4. Coordinates (Å) of the equilibrium structure of the gauche-conformer. Table S5. Principal derived bond lengths and angles from the MW results for the cis and gauche conformers. Table S6. The principal theoretical results for geometric parameters and molecular properties. The CCSD(T) calculations used 455 Cartesian basis functions. Only selected dihedral angles are shown; the atomic coordinates for each molecule, from which other dihedral angles can be generated is shown below. For ease of comparison, both conformers have the same numbering system as shown in Figure S1. Figure S1. The heavy atom partial skeleton derived from the microwave results, completed with calculated data for cis (4a), and gauche cyclopropyl (isocyanato)silane (4b). The inertial axis frames are indicated. The cis compound has CS symmetry and lies in the a,bplane. Figure S2. The full labelling used in the theoretical study. cis-cyclopropyl (isocyanato) silane (4a) showing the numbering system used in the structure determinations and shown



18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in the Tables of bonds and angles; the gauche conformer (4b) has the (H7,8)Si3N4C5O6 unit rotated about the C2Si3 axis. Figure S3. The set of highest occupied MOs for the cis and gauche conformers

References (1) Guirgis, G. A.; Wang, Z.; Lirjoni, J.; Palmer, M. H.; Obenchain, D. A.; Peebles, R. A.; Peebles, S. A. The molecular structure of difluoroisocyanato silane: A combined microwave spectral and theoretical study. J. Mol. Struct., 2010, 983, 5−11. (2) Guirgis, G. A.; Overby, J. S.; Palmer, M. H.; Peebles, R. A.; Peebles, S. A.; Elmuti, L. F.; Obenchain, D. A.; Pate, B. H.; Seifert, N.A. Molecular Structure of Methyldifluoroisocyanato Silane: A Combined Microwave Spectral and Theoretical Study. J. Phys. Chem. A, 2012, 116, 7822−7829. (3). Guirgis, G. A., Overby, J. S., Barker, T. J., Palmer, M. H., Pate, B. H., and Seifert, N. A., The Molecular Structure of Methylfluoroisocyanato Silane: A Combined Microwave Spectral and Theoretical Study, J. Phys. Chem. A, 2015, 119, 652−658. (4) Durig, J.R., Guirgis, G.A., Sawant, D. K., Seifert, N. A,. Deodhar, B. S., Pate, B. H., Panikar, S. S., Groner, P., Overby, J. S., Askarian, S. M., Microwave, r0 structural parameters, conformational stability andvibrational assignment of cyclopropylcyanosilane Chem. Phys., 2014, 445, 68–81.


Page 18 of 21

Page 19 of 21


19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5) Panikar, S. S., Guirgis, G. A., Eddens, M. T., Dukes, H. W., Conrad, A. R.; Tubergen, M. J.; Gounev, T. K.; Durig, J. R., Microwave, infrared and Raman spectra, adjusted r0 structural parameters, conformational stability, and vibrational assignment of cyclopropylfluorosilane, Chem. Phys., 2013, 415, 124-132. (6) Dakkouri, M.; Typke,V., The gas-phase molecular structure of cyclopropyltrifluorosilane as studied by electron diffraction and ab initio calculations, J. Molec. Struct., 1994, 320, 1328. (7) Simmons, H.E., Smith, R.D., A new synthesis of cyclopropanes, J. Amer. Chem. Soc., 1959, 81, 4256. (8) Guirgis, G. A., Pan, C., Bregg, J., Durig, J. R., Spectra and structure of silicon containing compounds. XLI. Infrared and Raman spectra, conformational stability, vibrational assignment and ab initio calculations of cyclopropylbromosilane, J. Molec. Struct., 2003, 657, 239–254.

(9) Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H., A broadband Fourier transform microwave spectrometer based on chirped pulse excitation, Rev. Sci. Instrum. 2008, 79 (5), 053103/1-053103/13 (10)

Pérez, C.; Lobsiger, S.; Seifert, N. A.; Zaleski, D. P.; Temelso, B.; Shields, G. C.;

Kisiel, Z.; Pate, B. H., Broadband Fourier transform rotational spectroscopy for structure determination: the water heptamer, Chem. Phys. Lett. 2013, 571, 1-15. (11)

Kraitchman, J., Determination of molecular structure from microwave spectroscopic

data., Am. J. Phys. 1953, 21, 17−24. (12)

Pickett, H. M. The fitting and prediction of vibration-rotation spectra with spin

interactions. J. Mol. Spectrosc. 1991, 148, 371−377. (13)

Watson, J. K. G. Simplification of the molecular vibration-rotation hamiltonian, Mol.

Phys. 1968, 15 , 479-90. (14)

Watson, J. K. G. In Handbook of High-resolution Spectroscopy; John Wiley & Sons,

Ltd, 2011.



20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(15) 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 D.01; Gaussian, Inc.: Wallingford, CT, 2009. (16) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A., Self consistent molecular orbital methods. XX. A basis set for correlated wave functions. J. Chem. Phys. 1980, 72, 650. (17) McLean, A. D.; Chandler, G. S. Contracted Gaussian basis sets for molecular calculations. I. Second row atoms, Z = 11−18. J. Chem. Phys. 1980, 72, 5639−5648. (18) Woon, D.E., and Dunning, T.H., Gaussian basis sets for use in correlated molecular calculations. III. The atoms aluminum through argon.,J. Chem. Phys. 1993, 98, 1358. (19) AIMAll (Version 15.05.18), Todd A. Keith, TK Gristmill Software, Overland Park KS, USA, 2015 (aim.tkgristmill.com) (20) Bader, R. F. W. Atoms in Molecules: A Quantum Theory, Clarendon Press: Oxford, U.K., 1990. (21) Biegler-König, F.; Nguyen-Dang, T. T.; Tal, Y.; Bader, R. F. W. Calculation of the average properties of atoms in molecules. J. Phys. B 1981, 14, 2739−2751. (22) Bader, R.F.W., Atoms in Molecules, Encyclopedia of Computational Chemistry, Edited by Schleyer P.v.R., et al, John Wiley and Sons, Chichester, UK, 1998, 1, 64-86. (23) Wiberg, K. B., Bent Bonds in Organic Compounds, Acc. Chem. Res. 1996, 29, 229-234. (24) Kochanski, E., and Lehn, J. M., The Electronic Structure of Cyclopropane, Cyclopropene and Diazirine. An ab initio SCF-LCAO-MO Study, Theoret. Chim. Acta (Berl.) 1969, 14, 281-304. (25) Kimura, K., Katsumata, S., Achiba, Y., Yamazaki, T., Iwata, S., Handbook of HeI Photoelectron Spectra of fundamental organic molecules, Halstead Press, New York, (1981) p54. (26) Pasinszki, T., Yamakado, H., and Ohno, K., Penning Ionization of CH3SCN, CH3NCO, and CH3NCS by collision with He*(23S) Metastable Atoms, J. Phys. Chem. 1993, 97, 1271812724. (27) Bent, H. A., An appraisal of valence-bond structures and hybridization in compounds of the first-row elements, Chem. Reviews, 1961, 61, 275-311.


Page 20 of 21

Page 21 of 21


21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents graphic


Molecular Structure of Cyclopropyl (Isocyanato) Silane: A Combined

Recommend Documents