Structural and Energetic Properties of HaloacetonitrileâBCl3

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Structural and Energetic Properties of Haloacetonitrile – BCl Complexes: Computations and Matrix-IR Spectroscopy 3

James Allan Phillips, Samuel J. Danforth, Nicholas J Hora, John R Lanska, and Anna W Waller J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09715 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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The Journal of Physical Chemistry

Structural and Energetic Properties of Haloacetonitrile – BCl3 Complexes: Computations and Matrix-IR Spectroscopy

James A. Phillips*, Samuel J. Danforth, Nicholas J. Hora, John R. Lanska, and Anna W. Waller

Department of Chemistry, University of Wisconsin - Eau Claire, Eau Claire, WI 54702

*Corresponding Author: email: [email protected] (p) 715-836-5399 (c) 715-864-3938 (f) 715-838-4979

ACS Paragon Plus Environment

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Abstract: The FCH2CN–BCl3 and ClCH2CN–BCl3 complexes have been investigated by quantum-chemical computations and low-temperature, matrix-isolation-IR spectroscopy. Theory predicts two stable equilibrium structures, with distinctly different B-N distances, for both complexes. One set of structures, which correspond to the global energy minima, exhibit B-N distances of 1.610 and 1.604 Å for FCH2CN–BCl3 and ClCH2CN–BCl3, respectively (via M06-2X/aug-cc-pVTZ). The corresponding binding energies are 5.3 and 6.3 kcal/mol. For the metastable structures, the B-N distances are 2.870 and 2.865 Å for FCH2CN–BCl3 and ClCH2CN–BCl3, respectively, and the corresponding binding energies are 3.2 and 3.3 kcal/mol. Also, the barriers between these structures on the B-N distance potentials are 2.5 and 2.8 kcal/mol, respectively, relative to the secondary, long-bond minima. In addition, several IR bands of both FCH2CN–BCl3 and ClCH2CN–BCl3 were observed in nitrogen matrices, but the assigned bands are consistent with M06-2X predictions for the shortbond, minimum energy structures. None of the observed IR bands could be assigned to the meta-stable, long-bond structures.


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Introduction

Investigations of the structural and bonding properties of molecular complexes span several decades,1-9 and within this vast body of literature, a large portion of our work has been concerned with nitrile – BX3 systems (X = H, F, Cl).8 The original structural characterizations of CH3CN–BCl3 and CH3CN–BF3 via crystallography10 and IR spectroscopy11 took place nearly 50 years ago. Nevertheless, even quite recently, a quantum-chemical study of HCN–MX3 (M=B, Al; X=H,F,Cl) compounds noted some distinguishing features as to how complex formation causes the electron density to shift differently in BF3 and BCl3, relative to its AlX3 and H-containing counterparts.12 In a broader sense, complexes involving Group 13 (or 3A) “MX3” acceptors have been classified as “πhole” complexes,13 because the electron-deficient region extends perpendicular to the primary molecular framework (i.e., out of the MX3 plane in these cases), in contrast to “σ-hole” systems13-15 such as H-bonds, halogen bonds, and complexes of Group 14 (or 4A) “MX4” acceptors, in which the electron deficient region lies in-line with, and opposite of, a polar bond within the acceptor.

The primary theme of our research has been exploring the extent to which bulk, condensedphase media induce structural changes in various types of donor-acceptor complexes,8, 16, 17 and identifying the underlying factors that manifest such effects.8, 18-20 For the nitrile - BF3 systems in particular,6, 8 interactions with a medium cause the donor-acceptor distance to contract markedly, and in turn, the acceptor sub-unit distorts to accommodate an additional bond, and the most direct illustrations of this phenomenon are gas-solid structure differences.6 For example, the B-N distance of HCN–BF3 is 2.47 Å in the gas-phase21 yet it contracts to a value of 1.65 Å in the crystalline solid.10 In our initial study of FCH2CN–BF3, we predicted a 0.78 Å difference in B-N distance between the measured, x-ray (R(B–N) = 1.64 Å) and calculated, gas-phase structures (R(B–N) = 2.42 Å, via B3PW91/aug-cc-pVTZ).22

The natural question that arises upon such observations is the extent to which a solvent or other bulk-phase medium may cause analogous structural changes. In this vein, we have determined via both experimental and theoretical evidence that inert gas matrices can also induce a significant and progressive contraction of the donor-acceptor bonds in various nitrile - BF3


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systems.8, 18, 20 In fact, our observations parallel those made by Andrews and co-workers for H3N– HCl;23 a complex in which matrix media dramatically affect the extent of proton transfer (as opposed to purely electron transfer in the Lewis acid - base systems). In the case of H3N–HCl, the red-shift on the H-Cl stretching band increases with an enhanced hydrogen bonding interactions across a series of increasingly polarizable matrix hosts: Ne < Ar-Ne (mix) < Ar < Kr.23 In our studies of analogous electron donor-acceptor systems, the most extreme results have been obtained for FCH2CN–BF3,20 for which we observed that the B-F asymmetric stretching frequency shifted systematically over a range of nearly 250 cm-1 across various matrices and other media, in a manner consistent with B-N bond contraction.24, 25 Specifically, the solid-state value was observed at 1186 cm-1, the gas-phase, B3PW91 prediction was 1429 cm-1,22 and the neon, argon, and nitrogen matrix frequencies lied between these extremes.20 Moreover, like the results for H3N–HCl, the red-shifts in these bands systematically paralleled the tendency of the medium to stabilize charge (i.e., Ne < Ar < N2).20 In turn, quantum-chemical investigations of the B-N potential of FCH2CN–BF3 revealed a broad, flat region ranging from 1.8 to 2.5 Å, in which the energy rises only about 2 kcal/mol between the equilibrium bond length (~2.5 Å) and the onset of the inner potential wall (~1.8 Å).20 Ultimately, we were able to reproduce the B-N bond contraction implied the matrix IR shifts by mapping these B-N potentials in dielectric media (PCM/M06/aug-cc-pVTZ)26 and noting that the inner region was preferentially stabilized by the bulk medium; the complex is more polar at shorter B-N distances, which amplifies the “solvent” interaction. The end result is that the equilibrium B-N distance shifts inwards, and the effect is systematic, becoming more extreme as the dialectic constant is increased.8, 19, 20

Subsequent to our work on nitrile – BF3 systems, we turned to CH3CN–BCl3, which also exhibited some unique structural and energetic properties, even if it was much less prone to medium effects.27 Initially, via MP2 and B3PW91 computations, we identified two distinct gasphase structures with different equilibrium B-N distances, each of which corresponded to a previous theoretical result.5, 28 The so-called “short-bond” structure, with a B–N distance of 1.60 Å, was found to be the global energy minimum, with a binding energy of 12 kcal/mol (via MP2/aug-ccpVTZ). This structure was similar to that reported previously by Cho and Cheong.28 A metastable structure was also predicted, with a much longer B-N distance (2.69 Å), and a binding energy of 4.9


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kcal/mol (again, MP2/aug-cc-pVTZ). This structure corresponded to results reported previously by Jonas et al.5 However, in our low-temperature experiments, we observed only IR bands attributable to the minimum-energy, “short-bond” form in neon matrices at 5 to 6 K, in spite of the fact that we predicted a significant (~ 2 kcal/mol) barrier along the B-N potential that separated these structures, and it persisted at fairly high levels of theory (e.g., CCSD/6-311+G(2df,p)).29

The relative Lewis acidity of BF3 and BCl3 is by no means a new subject, but we have found that there are key aspects unique to nitrile complexes, and in turn, that these manifest the peculiar B-N potentials that underlie the novel properties of these systems.8 The long-standing rationale for the greater acidity of BCl3 has been that back-donation of π-electrons from the halogen to the electron deficient boron atom is more extensive for B-F bonds, rendering the boron in BF3 less electron deficient than its counterpart in BCl3.30 More recently, several computational studies offered alternative explanations.31-33 One study noted that the B-X bonds in BF3 actually have slightly less πoverlap than those in BCl3, and that an increased charge capacity of BCl3, allows for greater charge transfer.31 Another study invoked ligand close-packing theory, which asserts that geometric distortion, including a lengthening of the B-X bonds that stems from repulsive interactions between the donor ligand and halogen substituents, is the primary energetic factor.32 A third study points simply to the increased covalent contribution to the bonding in BCl3 complexes due to a lower LUMO energy.33

However, these more recent studies explicitly contrasting BF3 and BCl3 deal exclusively with NH331, 33 and amine complexes;32 none of them address the Lewis acidity of BF3 or BCl3 toward nitriles, in which the donor orbital (approximately an “sp lone pair” ) is not only less extended than that in an amine donor, it is also crowded by C-N π-bonds that manifest additional steric repulsion with the acceptor. In our studies, which involved a series of nitrile - BX3 complexes (X=H, F, Cl),27, 34 we determined via energy decomposition analyses9 that the magnitude of these repulsive interactions varied greatly between nitrile complexes of BH3, BF3 and BCl3 due to the size of the X atom (and/or the extent of π-bonding). Though this seems reminiscent of the ligand close-packing rationale, 31 we found that the steric repulsion itself was much more significant than differences in geometrical distortion energy; not only both in terms of its relative contribution to the overall


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energy of each complex, but also in the incremental differences in the steric repulsion energy terms between the X = H, F, and Cl systems.29 And again, this effect is greatly amplified in nitrile complexes due to the nature of the sp-like donor orbital and π-electron density about it.34 The upshot is that the repulsive effect is minimal in nitrile - BH3 systems, thus they are strong complexes with extremely short B-N distances in the gas-phase (less than 1.6 Å).8, 34 In the case of nitrile – BF3 complexes, there is an intermediate degree of repulsion, and furthermore, this effect sets in at distances at which bonding interactions become significant. Thus, a net cancelation occurs, rendering the B-N potential flat and anharmonic, which leads ultimately to the extreme medium effects.8, 20 For CH3CN–BCl3, the repulsion is more significant, and sets in at longer distances, beyond those at which bonding interactions can overcome it. The result is a barrier at intermediate distances, and two distinct minima along the B-N potential curve.8, 27

In this manuscript, we present both experimental and theoretical results for two singlyhalogenated analogues of CH3CN–BCl3: FCH2CN–BCl3 and ClCH2CN–BCl3. The intent, presuming initially that two minima would persist on the B-N potentials of these systems, was to weaken the B-N interaction via halogen substitution on the nitrile, thereby raising the energy of the “shortbond” structure as to equalize the energies of the expected potential minima. Presumably, this would increase the likelihood that we would observe signals for both distinct structures in lowtemperature IR experiments. Below, we will report equilibrium structures, harmonic frequencies, and B-N potential curves obtained from both density functional and post-Hartree-Fock calculations, and indeed we do predict two distinct equilibrium structures for these systems akin to those observed for CH3CN–BCl3. We also observed several IR bands of FCH2CN–BCl3 and ClCH2CN–BCl3 in cryogenic matrices, and ultimately, a comparison between the observed frequencies and M06-2X predictions provides a clear indication of which structural forms were observed in the matrix samples.


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Methods

Computations All computations were performed using Gaussian 09 revision B.0.1.35 For equilibrium geometry and frequency calculations, optimizations were performed within Cs symmetry for the complexes, and convergence criteria were set using “opt=tight” option. This sets the maximum and RMS forces to 1.5 x 10-5 and 1.0 x 10-5 hartrees/bohr, respectively, and the maximum and RMS displacement to 6.0 x 10-5 and 4.0 x 10-5 bohr, respectively. An ultrafine integration grid was also utilized, and the aug-cc-pVTZ basis set 36 was used exclusively throughout this study.

Previously for CH3CN–BCl3,27 we obtained structures and frequencies using MP2 and two hybrid density functional methods (mPW1PW91 and B3PW91),36 but in the absence of an experimental gas-phase structure upon which to validate these results and a marginal agreement between the MP2 and DFT structural results, we ultimately reported results from both MP2 and mPW1PW91. In this present study, initial computations were performed using those methods, but subsequently, we explored three additional density functional methods that have performed well in our more recent work,16, 17 specifically M06,37 M06-2X,37 and ω-B97X-D,38 with the intent of identifying a single method suitable for both structure and frequency predictions. Of these, we found that M062X provided the best performance because: i) It provided the best agreement with the MP2 structure predictions for the complexes (DFT and MP2 results are significantly more consistent than in our previous work27), and ii) it provided the most accurate frequency predictions for free-BCl3; with an RMS error of 6.9 cm-1 relative to experimental values,39, 40 which was significantly better than MP2 (RMS error = 12.9 cm-1). The rationale for the latter criterion is that the structurallysensitive vibrational modes involve motions within the BCl3 moiety,25, 27 and thus an accurate representation of the BCl3 force field is essential for the structural interpretation of the experimental IR spectra. As such, we will emphasize the M06-2X results below, but do note analogous MP2 results in the text for reference. Atomic charges were also computed, using the Natural Population Analysis (NPA) procedure,41 and this was executed directly from within Gaussian.


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The B-N distance potentials were mapped in a point wise manner over a range of 1.4 to 3.6 Å in 0.1 Å steps, with additional points at 1.55 and 1.65 Å to make the curves appear less jagged in the region of the global minima. All degrees of freedom aside from the B-N distance were optimized at each point along these curves, within the Cs-symmetry constraint, though in a few instances the final structures exhibited very slight deviations from full symmetry. In this phase of the study, we also explored the performance of several implementations of DFT,36 including: B3PW91, B972, M06,37 M06-2x,37 and ω-B97X-D,38 and also computed MP2 curves and CCSD curves based on the MP2 structures (i.e., CCSD/aug-cc-pVTZ//MP2/aug-cc-pVTZ). Below we display the M06-2X and post-Hartree-Fock results, but have included curves for the other DFT methods as supporting information. We note here that the M06-2X potentials exhibit excellent agreement with CCSD/MP2 over the range of the curve, far superior to MP2, which provides some additional justification for focusing on the M06-2X structure and frequency results.

Materials BCl3 (99.9 %) was obtained from Sigma-Aldrich. In order to minimize signals due to HCl and other unknown impurities in the IR spectra, it was purified by condensing about 3 mL in a liquid sample tube attached to the preparatory vacuum line, and subjected to several freeze–pump–thaw cycles and/or brief periods of continuous evacuation at 77 K and/or 196 K. We note here that the BCl3 was extremely unstable, and apt to decompose and/or hydrolyze with even trace amounts of water, especially when exposed to metal surfaces. Dry conditions, regular re-purification, and the use of Teflon plumbing where ever possible were essential for successful experiments with BCl3. FCH2CN (98 %) and ClCH2CN (99 %) were also obtained from Sigma-Aldrich but were subjected to 3 or 4 freeze-pump-thaw cycles before use. N2 and Ar (both 99.9999 %) host gases were obtained from Praxair and used without further purification.

Matrix Isolation IR Spectra IR spectra were collected using two previously described matrix isolation apparatuses. Initial studies were conducted using a 10K system17, 42 that utilizes a Cryomech ST15 optical cryostat and a


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Nicolet Avatar 360 FTIR spectrometer. Spectra are collected on this system in transmission mode; the matrix sample is deposited on a KBr window, and temperature is maintained using a Scientific Instruments 9600-1 temperature controller with a single Si diode located at the end of the second refrigeration stage. Subsequent experiments were conducted on a 4K system18, 27 that employs a Janis SHI-4-5 optical cryostat and a Thermo Nexus 670 FTIR. With this system, spectra are collected in reflection mode; the matrix is deposited on a gold mirror and an external IR beam is focused onto a gold sample mirror, and reflects at an angle of approximately 90° before it is collected and refocused onto a DTGS detector. Sample temperature is maintained using a Lakeshore 331 temperature controller with two Si diodes, one directly behind the sample mirror and another at the end of the second refrigeration stage. Two advantages of this system are that a great deal of the matrix deposition plumbing is composed of Teflon tubing, and in addition, these lines are fitted with high-throughput bypasses leading directly to the main pumping system, and active pumping (when not in use) minimizes ambient water and other impurities in the spectra. Both systems are pumped by corrosive-compatible turbomolecular pumps (Pfieffer), which are backed with rotary vane pumps employing fluorocarbon oil (Pfeiffer).

Matrix-IR samples were prepared by making separate gas mixtures containing either nitrile or BCl3, in N2 or Ne, in 2 L glass bulbs, on preparatory a gas manifold (glass, with Teflon stopcocks), which is evacuated with a glass/oil diffusion pump (Chemglass). Individual guest-host ratios ranged from 1/400 to 1/1600 (nitrile or BCl3 / N2 or Ne). To prepare matrix samples, the mixtures were flowed simultaneously thru Granville-Phillips #203 leak valves, into the vacuum chamber an onto the sample window, usually at equal rates, thereby reducing the actual guest concentrations to half their initial, prepared value. The exceptions were the control (single guest) experiments in which only one mixture was flowed. Optimum signals for the complexes were obtained with a matrix composition of 1/2/1600 (BCl3/nitrile/host). Matrix samples were deposited at temperatures of 12 to 21 K for nitrogen matrices, or 5 to 6 K for neon matrices, at flow rates typically ranging from 3 to 10 mmol/hour. Most samples were annealed to facilitate the identification of complex peaks (2530 K for N2, 8-9 K for Ne). IR spectra were recorded at 1 or 2 cm-1 resolution, and typically 200 to 400 scans were averaged.


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Results and Discussion

Equilibrium Structures Because of our previous results of CH3CN–BCl3,27 we were expecting two distinct stable structures with different B-N distances for both FCH2CN–BCl3 and ClCH2CN–BCl3. As such, we took care to use two distinct sets of starting structures in our initial equilibrium geometry searches, with B-N distances of approximately 1.6 and 3.0 Å. At the outset of this process, we used both MP2 and B3PW91, but as noted above, we subsequently used a few more recently-developed DFT methods (M06,37 M06-2X,37 and ω-B97X-D38). In the end, all methods produced reasonably consistent structural results, and though the values of the binding energies varied to some extent (see below), they all predicted two distinct structures, with B-N distances of about 1.6 Å (the “short-bond” form henceforth), or about 2.9 Å (the “long-bond” form henceforth). The M06-2X structures are displayed in Figure 1, and in addition to the rationale offered above for this choice, we note that the M06-2X values for the binding energy agree quite well with the CCSD/MP2 values noted in the B-N potential curves below (superior to MP2 and most of the other DFT methods), which offers additional support for choosing to focus on these results.

We conducted a systematic search for the equilibrium conformer of each complex by calculating geometries, frequencies, and binding energies for both eclipsed and staggered forms of both the “short” and “long” structures of each complex. These systems are slightly bent along the B-N-C-C linkage, thus the “eclipsed” and “staggered” labels do not apply in a formal sense. However, in the eclipsed case, the carbon - halogen bond of the nitrile lies in the plane of an adjacent B-Cl bond, essentially “cis” (note the ClCH2CN–BCl3 structures in Figure 1). Conversely, in the “staggered” conformer, the nitrile C-X bond is co-planar with a B-Cl bond on the opposite side of the primary B-N-C-C linkage, essentially “trans” (note the FCH2CN–BCl3 structures in Figure 1). The upshot of the conformational analysis is that the complexes have nearly free rotation about the torsional degree of freedom, and there are only negligible differences in key structural parameters or experimentally accessible frequencies between the two forms, and the energy differences between them are arguably less than the precision of the computations.36


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Figure 1: Calculated equilibrium structures and binding energies for FCH2CN–BCl2 and ClCH2CN–BCl3 via M06-2X/aug-ccpVTZ. The B-N distances via MP2/aug-cc-pVTZ are and 1.619 Å and 2.793 Å for the “short” and “long” forms of FCH2CN– BCl3, and 1.613 Å and 2.770 Å for the “short” and “long” forms of ClCH2CN–BCl3, respectively.


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Nonetheless, the situation for the ClCH2CN–BCl3 is clear; the eclipsed form is lower in energy, and the torsional frequency is real for the eclipsed geometry but imaginary in the staggered case. These conditions apply to both short and long forms, and with both M06-2X and MP2. As such, we based our subsequent analysis on the eclipsed form of ClCH3CN–BCl3. However, we do note that the energy differences between the eclipsed and staggered forms are slight, specifically 0.07 kcal/mol and 0.01 kcal/mol via M06-2X for the short and long forms, respectively. For FCH2CN–BCl3 the results are ambiguous. The eclipsed conformation is lower in energy for both short and long forms (by a margin even less than that for the ClCH2CN complex), but the torsional frequencies are imaginary. Conversely, the torsional frequencies of the staggered from are real, and again the results for M06-2X and MP2 are completely consistent. Thus, for the FCH2CN complex, we decided to report the structure and frequencies for the staggered structures, but computed the B-N potential for the eclipsed form, it being the minimum energy form throughout the range of the curve, presumably. We note also that the energetic preference for the eclipsed conformations is consistent with recent results from study of halogenated 2-butynes (X3C-CC-CX3; X=H, F, Cl, Br, I, At). For the l-, Br-, and I-containing systems in this series, the eclipsed conformation is 0.05 to 0.40 kcal/mol lower in energy than the staggered form.43 The B-N complexes in the present study, the short-bond forms in particular, are essentially B-N analogs of the Cl-containing 2-butynes, albeit with only a single halogen (F or Cl) on the opposite end.

Anyhow, we note again that we identified two distinct equilibrium structures for both complexes, and these are displayed in Figure 1. We also note that these geometries are generally consistent with those we observed previously for CH3CN–BCl3.27 In the case of the short-bond forms, which correspond to the global energy minima, the structures resemble systems with reasonably strong B-N coordinate bonds. The B-N distance (via M06-2X) for FCH2CN–BCl3 is 1.610 Å, and the N-B-Cl angles range from 103.8 to 104.9°, only a few degrees less than the ideal tetrahedral value. The B-N distance in the short-bond form of ClCH2CN–BCl3 is 1.604 Å, and the N-B-Cl angles range from 104.6 to 104.8°, and these data convey an interaction that is just slightly stronger than in the case of the FCH2CN complex. For comparison, we note that the B-N distance in CH3CN–BCl3 (s) is 1.562 Å 10, which is about 0.04 Å shorter than the MP2/aug-cc-pVTZ result we reported previously (1.601 Å),27 as well as that in an earlier computational study.28 However, the binding


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energies, 5.3 kcal/mol and 6.3 kcal/mol respectively for FCH2CN–BCl3 and ClCH2CN–BCl3, are significantly smaller in magnitude than some strong, and directly related B-N complexes with similar bond distances. For example, the MP2/cc-pVTZ value for H3N–BCl3 is 29.1 kcal/mol (R(B-N) = 1.628 Å) ,and that for FCH2CN–BH3 via MP2/aug-cc-pVTZ is 20.3 kcal/mol (R(B-N) = 1.581 Å). Even the value previously reported MP2/aug-cc-pVTZ value for the short-bond from CH3CN–BCl3 (12.0 kcal/mol) is notably larger. So, in spite of the short B-N distances predicted for FCH2CN–BCl3 and ClCH2CN–BCl3, and corollary distortion of the BCl3 subunits in these systems, the B-N coordinate bonds are fairly weak in comparison to related systems, and we also note that, as expected, the FCH2CN complex is slightly weaker than the ClCH2CN system. In any event, a significant degree of charge transfer is predicted for both complexes by an analyses of NPA atomic charges;5, 41 the net charge on the BCl3 subunits in these complexes are -0.31e and -0.33e for FCH2CN–BCl3 and ClCH2CN–BCl3 respectively, which indicate a net transfer of about a third of an electron in each case.

By contrast, the meta-stable long-bond structures resemble weakly-bonded systems by any measure. The B-N bond distances (via M06-2X) for these structures are 2.870 and 2865 Å for FCH2CN–BCl3 and ClCH2CN–BCl3, respectively. These results are comparable to the value for the long-bond form of CH3CN–BCl3 (2.687 Å via MP2/aug-cc-pVTZ, 3.099 Å via mPW1PW91/aug-ccpVTZ).27 In terms of an experimental comparisons, we note that the B-N distances is the weaklybonded systems N2–BF3 and NCCN–BF36 have B-N distances 2.875 Å44 and 2.647 Å,45 respectively. The energies of the secondary minima for the FCH2CN and ClCH2CN complexes are comparable to one another, with energies of -3.2 and -3.3 kcal/mol, respectively, relative to the separated nitrile and BCl3 fragments. These values are comparable to the CCSD/6-311+G(2df,2pd) value for CH3CN– BCl3 (-3.6 kcal/mol). Additionally, the MP2/aug-cc-pVTZ, energy values for these minima, again relative to the separated fragments, are -4.3, -4.5, and -4.9 kcal/mol, respectively, for FCH2CN–BCl3, ClCH2CN–BCl3, and CH3CN–BCl3. These data indicate that the effect of the halogen on the energy of this secondary potential minimum is significantly less than the analogous effect on the global minimum, and thus, as was our intent at the outset of this study – halogen substitution acts to partially equalize the two minima along the B-N potential. Finally, we note that the degree of


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charge transfer implied by the NPA charges5, 41 is negligible; the net charge on the BCl3 subunit for both FCH2CN–BCl3 and ClCH2CN–BCl3 is -0.014e.

Donor - Acceptor Potentials Figure 2 displays boron-nitrogen potential energy curves computed via M06-2X, MP2, and CCSD//MP2 (with the aug-cc-pVTZ basis set), which convey the (electronic) energy along the donoracceptor reaction coordinate, relative to that the of the separated fragments. Analogous curves from several additional DFT methods are included as supporting information (Figure S1). One rather striking observation is the agreement between the M06-2X curves and the CCSD//MP2 curves, which is generally superior to the other methods, including MP2, especially at longer, nonbonded B-N distances. We do note that M06 energies agree quite well with CCSD//MP2 for the inner half of the curve, as do the HSEh1PBE energies near the global minimum. However, these methods also seem to imply a shorter equilibrium B-N distance (for the inner minimum). In a boarder sense, the utility of these B-N curves is that they not only provide a visual illustration of the relative energies of the two equilibrium structures, but also convey information regarding the barrier between them. For FCH2CN–BCl3, the potential energy maximum for the barrier occurs at 2.1 Å, and the energy is -0.3 kcal/mol relative to the separated fragments (via M06-2X of CCSD/MP2). Thus, maximum lies about 2.8 kcal/mol above the outer, secondary potential minimum. For ClCH2CN–BCl3, the barrier occurs between 2.1 and 2.2 Å, and the energy is about -0.8 kcal/mol, or about 2.5 kcal/mol above the outer minimum.


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A) FCH2CN–BCl3

B) ClCH2CN–BCl3

Figure 2: Calculated B-N distance potentials (electronic energy vs. B-N distance) for FCH2CN–BCl2 and ClCH2CN–BCl3 via M06-2X, MP2, and CCSD//MP2, using the aug-cc-pVTZ basis set. The zero of energy is set to that of the separated fragments. Curves from additional methods are available as supporting information.

The M06-2X curves for FCH2CN–BCl3 and ClCH2CN–BCl3 are re-plotted with the analogous curve for CH3CN–BCl3 in Figure 3, which illustrates the effect of halogen substitution on the overall energetic features of the donor-acceptor reaction coordinate. The result is a significant weakening of the interaction; the (relative) energies of the halogenated complexes are higher across the entire range of the curves. However, the effect is greater in the inner region, near the global, “bonded” minimum, and thus less significant near the secondary, “non-bonded” minimum; halogenation seems to weaken the bonding interactions more so than the long-range, non-bonding forces. The result is the energies of the “long-bond” and “short-bond” minima are somewhat equalized for the halogenated complexes, relative to the case of CH3CN–BCl3, which is an effect we sought. Also, we note that the barrier is lower on the CH3CN–BCl3 curve, both in terms of overall energy relative to the corresponding fragments (-2.4 kcal/mol at 2.2 Å), but also relative to the long-bond minimum (1.4 kcal/mol). Nonetheless, the barriers on all these curves far exceed kT at cryogenic temperatures typical of matrix isolation experiments (e.g., 0.036 kcal/mol at 18 K). We again note that these barriers persist at high levels of both wave function and density functional theory. Furthermore, in the case of CH3CN–BCl3,8, 27 energy decomposition analyses9 provided a sound


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physical rationale for its occurrence. Thus, the barriers do not seem to be theoretical artifacts, and thus it is reasonable to expect that one could observe both forms of these systems at cryogenic temperatures.

Figure 3: A comparison of B-N distance potentials for FCH2CN–BCl3, ClCH2CN–BCl3, and CH3CN–BCl3, computed via M06-2X/aug-cc-pVTZ. The zero of energy is set to that of the separated fragments.

IR Spectra and Frequencies We recorded IR spectra of complexes in low-temperature matrices, and focused primarily on nitrogen matrix experiments, but did conduct a few investigations of FCH2CN–BCl3 in neon matrices as well. For both FCH2CN–BCl3 and CCH2CN–BCl3, we observed a series of product bands (those requiring the presence of both BCl3 and nitrile), and assignments were initially guided by M06-2X predictions, and confirmed by isotopic shifts (whenever possible), a tendency to grow upon annealing, and consistent relative intensities across a range of sample compositions. In some cases, however, peak area measurements were obscured by overlapping signals, especially in the case of FCH2CN–BCl3, for which product bands sometimes overlapped peaks due to FCH2CN, or impurities and decomposition or hydrolysis products from within the BCl3 sample

In the case of nitrogen matrices seeded with ClCH2CN and BCl3, we observed product bands at 525, 642, 675, 777/787, 809/817 and 2366 cm-1 (groups of frequencies, separated by /’s, were observed as adjacent, partially overlapped doublets). Representative spectra are displayed in Figure 4. Focusing first on the set of signals observed in the 770 – 820 region (Figure 4B), which lie


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just below the prominent signals due to the asymmetric stretching ( ) bands of BCl3 (985 and

945 cm-1). These signals are not only rather intense, but also quite broad due to the mixed 35Cl-37Cl isotopic composition of the BCl3 sample. We expect to observe the asymmetric stretching bands of the ClCH2CN–BCl3 complex to occur in the region lying just below the bands of free BCl3, thus

the pairs of doublets at 777/787 and 809/817 are good candidates for this assignment. Indeed, they exhibit a characteristic 4:1 intensity ratio (respectively, based on peak areas) consistent with relative natural abundance of boron isotopes (11B/10B = 4/1). Moreover, the predicted M06-2X frequencies for the bands are 796/789 cm-1 for 11B and 829/822 cm-1 for 10B (the mode is

doubly degenerate in BCl3, but splits slightly due to the lower, Cs symmetry of the complex). Thus, the observations agree marginally well with the predictions; the differences in the absolute frequency values could reflect a slight compression of the B-N bond in the matrix relative to the predicted gas-phase structure. Nonetheless, the isotope shifts (30-32 cm-1 observed and 33 cm-1 predicted) agree incredibly well, thus we assign these 777/787 and 809/817 bands to the

modes of ClCH2CN–11BCl3 and ClCH2CN–10BCl3, respectively. The doublet splittings we observe do exceed those of the predictions, and for some unknown reason the lineshape of each doublet component is somewhat different. Initially, this led us to believe that the lower-frequency component may be due to a larger cluster, but the areas remain reasonably consistent in various experiments and upon annealing. Thus, we presume that the doublet is due to trapping sites, and the in most instances we simply fail to resolve the nearly degenerate components of the band,

though we occasionally did observe two partially resolved maxima on the 787 cm-1 peak.

The assignment of the other product bands follows directly from the conformation of the bands. The group of peaks in the 640 to 680 region (Figure 4A), lies near the M06-2X frequency predictions for the BCl3 symmetric deformation ( ) or “umbrella” mode (645 and 671

for 11B and 10B, respectively). Both the peaks at 642 and 675 cm-1 exhibit partially resolved shoulders to the low-frequency side, at about 629, and 660 cm-1, respectively. These splittings are somewhat similar to what is observed for the bands discussed above. Because the area ratios

of these peaks at 642 and (with or without the shoulders included) exhibit a 4:1 Intensity ratio, and the splitting is consistent with the prediction as well as the predicted isotope shift, we confidently


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assign the bands observed at 642/629 and 660/675 to the bands of ClCH2CN–11BCl3 and

ClCH2CN–10BCl3, respectively. A)

B)

C)

Figure 4: IR spectra of nitrogen matrices containing: i) BCl3 1/800, 18 K ii) ClCH2CN 1/400 @ 18 K, iii) ClCH2CN/BCl3 1/2/800 @ 18 K, iv) ClCH2CN/BCl3 1/2/800, annealed at 27 K; bottom to top, respectively, in each plot (A-C). Peaks assigned to ClCH2CN–BCl3 are noted with *’s.


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The remaining product bands are assigned on the basis of consistent relative intensity ratios, based on peak areas, as well as theoretical predictions. The product band at 2366 cm-1 (Figure 4C) is a good candidate for the C-N stretching mode ( ), and in addition to the intensity ratios, we note that it lies near the frequency observed for the short-bond form of CH3CN–BCl3 (2380 cm-1). The M06-2X prediction of this band is 2512 cm-1, but in most of our past work,8, 25 our predictions for have greatly overestimated the experimental values in the absence of a scaling factor. As such we assign the peak at 2366 cm-1 to the band of ClCH2CN–BCl3 (no B isotope shift is predicted), and we will explore the effect of a scaling factor below. In addition, the product band 525 cm-1 agrees somewhat with the predictions C-C-N bending motion (556 cm-1, for the shortbond structure, or 513 cm-1 for the long bond form), so we offer that as a tentative assignment.

These frequency assignments for ClCH2CN–BCl3 are summarized in Table 1, with predictions for both short- and long-bond structures. A complete set of frequencies for both structures is included as supporting information (Table S2). It is clear, however, the frequencies we observe, at least for the BCl3-localized modes, agree reasonably well with the predictions for the short-bond, global minimum energy structure of ClCH2CN–BCl3. In the case of the band, if we apply a scale factor of .958 (determined from frequencies for free ClCH2CN by taking the ratio of the observed matrix value to the M06-2X prediction, i.e., νmatrix/νM06-2X), we predict the band of the short complex to occur at 2406 cm-1. While this agreement is still far from stellar, a similar scaling of the long-bond prediction for the band renders a value of 2333 cm-1, just 5 cm-1 to the red of the observed band for free ClCH2CN. As for the band, we note that the M06-2X prediction (507 cm-1) exceeds the observed frequency (485 cm-1) by 22 cm-1, which comparable to the difference between the observed band and the short-bond prediction. Thus, we can account for the seemingly anomalous and frequencies (the model validation was based on BCl3), and can confirm only the occurrence of the short-bond form of the complex in the nitrogen matrix environment.


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Table 1: Observed and calculated vibrational frequenciesa for ClCH2CN–BCl3 Calc. (long)b

Exp. (N2)c

Calc. (short)b

C-C-N Bend ( )d

513

525

556

11

B - BCl3 "Umbrella" ( )

419

642/629

646

10

B - BCl3 "Umbrella" ( )

437

675/660

671

11

B - BCl3 Asy. Stretch ( )

796/789

787/777

967/958

10


829/822

817/809

1008/998

2436

2366

2512

Mode

C-N stretch ( )

a) Units of cm-1. b) Harmonic frequencies calculated via M06-2X/aug-cc-pVTZ. c) The experimental uncertainty is ±2 cm-1. d) 11B value - a small 10B-11B isotope shift is predicted in some instances.

Frequency assignments for the product bands observed in nitrogen matrices containing FCH2CN and BCl3 follow a similar rationale to those for ClCH2CN–BCl3 discussed immediately above. Representative spectra are displayed in Figure 5, and we note that we observed product bands at 567, 581, 658/676/678/696 (an overlapped quartet), 787/760, 825/810, 1072/1077, and 2376 cm-1. Of these, we assign the two pairs of bands at 787/760 cm-1 and 825/810 cm-1 (Figure 5B) to the 11B and 10B components of the bands of FCH2CN–BCl3. This is based on the isotopic signature, i.e.,

the 27 cm-1 10B /11B shift, and a nearly 4:1 peak area ratio, which is partially affected by overlap between the 787 cm-1 signal and a peak in the FCH2CN reference spectrum. The M06-2X calculations predict a pair of frequencies 798 and 794 cm-1 for the band(s) of the 11B

isotopomer of FCH2CN–BCl3 (short-bond form). While this absolute frequency agreement is fair, it again may reflect a slight matrix-induced compression of the B-N bond, relative to the predicted gas-phase structure. In any event, the 10B bands are predicted to lie at 827/832 cm-1, and once


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again, the predicted 10B /11B isotope shift (28-29 cm-1) compares favorably that that observed (27 cm-1).

A)

B)

C)

*

Figure 5: IR spectra of nitrogen matrices containing: i) BCl3 1/800, 18 K ii) FCH2CN 1/400 @ 18 K, iii) FCH2CN/BCl3 1/2/800 @ 18 K, iv) FCH2CN/BCl3 1/2/800, annealed at 27 K; bottom to top, respectively, in each plot (A-C). Peaks assigned to FCH2CN–BCl3 are noted with *’s.


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We were initially confused by the absence of an isotopic signature attributable to the

“umbrella” band for FCH2CN–BCl3, but the predictions (i.e., the displacement vectors) indicate a significant coupling between this motion and the C-C-N bend ( ). For the 11B isotopomer the peak associated with these motions are predicted to occur at 598 and 656 cm-1, while the analogous 10B predictions are at 606 and 674 cm-1. The lower-frequency members of these pairs agree only marginally with the 581 and 567 cm-1 bands (Figure 5A). Meanwhile, the higherfrequency bands agree favorably with the 656 and 674 cm-1 bands compare remarkably with the quartet observed at 658, 676, 678, and 696 cm-1. The predicted isotope shift (18 cm-1) compares quite well, if one views this group as two pairs of 10B-11B bands (658/678 and 676/696), and though the peak areas are not completely consistent with the interpretation, these peaks are weak, partially overlapped, and somewhat obscured by peaks visible in the reference spectra. As such, we assign these peaks as follows, in a manner that dismisses the coupling for simplicity sake, but we will clarify this with a footnote in the table below. We will assign the 581 and 567 cm-1 bands to the mode in FCH2CN–BCl3 (an 11B-10B isotope shift of 9 cm-1 is predicted), and we will assign the pairs at 658/678, and 676/696 to the modes of the 11B and 10B isotopomers of FCH2CN–BCl3,

respectively.

Again, we see a lone peak in the C-N stretching region at 2376 cm-1, which we assign to the

band of FCH2CN–BCl3 (no isotope shift predicted). Again, this agrees poorly with the M06-2X prediction (2512 cm-1), even after scaling by a factor of .957 (determined in an analogous manner as above for the scaled ClCH2CN–BCl3 prediction for this band) the result is 2404 cm-1. Again, the agreement is at best marginal, but the observed frequency is much more consistent with the shortbond prediction than that for the long-bond form, for which a mere 7 cm-1 shift from the free FCH2CN band is predicted. In addition, we observe a doublet at 1072/1077 cm-1, just to the blue of the prominent of stretching band of free FCH2CN, which makes it a good candidate for the band in the complex. The M06-2X calculations predict a frequency of 1159 cm-1 for this band, again a substantial over estimate, but the predicted value for free FCH2CN is 1135 cm-1, and the implied shift (25 cm-1) is near what we observe in the spectra (22 cm-1). As such, we assign the 1159 cm-1 to the band of FCH2CN–BCl3. We note that the prediction for the long-bond form is 1138 cm-1,


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which predicts a shift of only 3 cm-1, and thus this frequency does exhibit some structural sensitivity, in spite of the fact that the C-F bond is somewhat distanced from the B-N bond.

These frequency assignments are summarized in Table 2, together with predictions for both short- and long bond forms of FCH2CN–BCl3, and a complete set of M06-2X predictions is included as supplementary material (Table S2). A comparison of the data in Table 2 make it clear that the observed set of product bands is entirely consistent with the occurrence of the short-bond form of FCH2CN–BCl3 in the nitrogen matrix environment.

Table 2: Observed and calculated vibrational frequenciesa for FCH2CN–BCl3 Calc. (long)b

Exp. (N2)c

Calc. (short)b

C-C-N Bend ( )d,e

588

581

598

11

B - BCl3 "Umbrella" ( )e

421

678/658

656

10

B - BCl3 "Umbrella" ( )e

439

696/676

674

11


966/958

787/760

798/794

10


1006/998

826/810

832/827

C-F stretch ( )

1138

1077/1072

1159

C-N Stretch ( )

2439

2376

2512

Mode

a) Units of cm-1. b) Harmonic frequencies calculated via M06-2X/aug-cc-pVTZ. c) The experimental uncertainty is ±2 cm-1. d) 11B value - a small 10B-11B isotope shift is predicted in some instances. e) These descriptions are very approximate; the and motions are significantly coupled in the short-form of this complex.

In addition, with the intent of observing peaks attributable to the long-bond form of FCH2CN– BCl3, we did perform a few experiments in solid neon at 5-6 K, presuming that the lower temperature would increase the likelihood of observing the meta-stable structure. However, in these experiments, we observed a series of product band only marginally shifted from those we


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observed in nitrogen matrices, specifically at frequencies of: 580, 621/638/653/674, 768, 810, 1082 and 2374 cm-1. We did not perform enough experiments to conduct a meaningful analysis of intensity ratios like that noted above but the overall pattern agrees with the nitrogen-matrix data as well as the short-bond frequency predictions. We did, in a few of these experiments, observe a band for the long bond peak at low frequencies near 426 cm-1, near the prediction of the

form of FCH2CN–BCl3, but this signal was not completely reproducible; it lies in a noisy region near the low-frequency limit of the instrument, and seemed to correlate with high concentrations of HCl. Moreover, we did not see any additional peaks that agree with those predicted for the metastable structure; we would expect a peak just to the blue of the peak for free FCH2CN, but band indicate that it would red-shift only slightly from the band of the predictions for the

free BCl3. Thus, the complex peak would be obscured by the free BCl3 signals in the spectra, which are particularly broad due to the mixed isotopic composition.

In the end, our efforts to observe the meta-stable, long-bond structures in these and earlier27 experiments were hampered by several factors. For one, H-bonded nitrile complexes, formed from impurity HCl resulting from BCl3 decomposition and/or hydrolysis, have frequency shifts that are comparable to the long-bond BCl3 systems.27, 46 Thus, the nitrile-based vibrational bands would be difficult to distinguish in the presence of any significant HCl impurity. As for observations of the BCl3-localized modes, the mixed isotopic composition of BCl3 (noted above) makes the strong BCl3 asymmetric stretching band of un-complexed BCl3 quite broad, such the predicted shift for the analogous long-bond complex bands does not surpass the width of the free BCl3 peak. The other BCl3-localized bands lie at low frequencies, near the 400 cm-1 lower limit of most standard IR spectrometers, and moreover, there is often significant noise in this low frequency region. None of these challenges are completely insurmountable, and it may yet be possible to observed peaks under the right conditions. However, an alternative approach may be more suitable, such as jet cooled rotational spectroscopy. However, this approach would manifest a new set of difficulties; beyond the chemical challenges of preparing these complexes in an expansion of some form, the spectra would be quite congested and thus difficult to assign and interpret.

Summary and Conclusions


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We have explored the structural and energetic properties of FCH2CN–BCl3 and ClCH2CN–BCl3, via quantum-chemical computations and low-temperature matrix-IR experiments. For both FCH2CN–BCl3 and ClCH2CN–BCl3, we find two equilibrium structures with distinctly different B-N distances. The “short-bond” bond structures are the global energy minima, and exhibit B-N distances of about 1.6 Å, comparable to many B-N strong donor-acceptor complexes. However, the M06-2X/aug-cc-pVTZ binding energies of FCH2CN–BCl3 and ClCH2CN–BCl3 are 5.3 and 6.3 kcal/mol, respectively, which are significantly less than most complexes with relatively short B-N distances. By contrast, the long-bond structures have B-N distances of about 2.9 Å with binding energies of about 3 kcal/mol, and thus resemble weakly-bonded B-N systems. Scans of the B-N distance potentials indicate that the barriers between the long- and short-bond structures are 2.5 and 2.8 kcal/mol for FCH2CN–BCl3 and ClCH2CN–BCl3, respectively, relative to the secondary, long-bond minima. These complexes, especially in regard to the short-bond forms, are significantly weaker than CH3CN–BCl3, and FCH2CN–BCl3 is slightly weaker than ClCH2CN–BCl3. However, the secondary, long-bond minima for FCH2CN–BCl3, ClCH2CN–BCl3 and CH3CN–BCl3 are much nearer in energy than the collective set of global minima for these systems. Therefore, one result of halogenation is that the energies of the distinct minima on the respective curves are equalized to some extent, relative to CH3CN–BCl3. We had presumed this effect would increase the likelihood that the meta-stable long-bond forms would be observed in low-temperature IR experiments. Ultimately however, we observed several IR bands attributable to the complexes in nitrogen matrices, but the definitive peak assignments correspond only to M06-2X predictions for the short-bond structures for both for FCH2CN–BCl3 and ClCH2CN–BCl3. Thus, while we can certainly verify the presence of the minimumenergy, short-bond forms of these complexes in the cryogenic matrices, we observed no definitive signals for the meta-stable long bond forms, even in neon matrices at 5-6 K.

Supporting Information for Publication Additional B-N potential curves from alternate methods and a complete listing of M062X/aug-cc-pVTZ for both equilibrium structures of both complexes.


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Acknowledgments The work was supported by the National Science Foundation grants CHE-1152820 and CHE1566035 (to J.A.P.) as well as CHE-1229354 and CHE-1662030 (MERCURY Consortium). Additional support was obtained from the Petroleum Research Fund (53066-UR6), administered by the American Chemical Society, and the University of Wisconsin-Eau Claire, by the Office of Research and Sponsored Programs and via the acquisition and maintenance of the Blugold Supercomputing Cluster. J.A.P. would also like to express his gratitude to Professor Lester Andrews for offering his guidance and encouragement during a visit to UW-Eau Claire in the early years of the Phillips group.

References 1. Rose, J., Molecular complexes. First Ed.; Pergamon Press: Oxford ; New York, 1967. 2. Mulliken, R. S.; Person, W. B., Molecular Complexes: A lecture and reprint volume. WileyInterscience: New York, 1969. 3. Foster, R., Molecular Complexes. Crane, Russak: New York, 1973. 4. Yarwood, J., Spectroscopy and Structure of Molecular Complexes. Plenum Press: London ; New York, 1973. 5. Jonas, V.; Frenking, G.; Reetz, M. T., Comparative theoretical study of Lewis acid-base complexes of BH3, BF3, BCl3, AlCl3, and SO2. JACS 1994, 116, 8741-8753. 6. Leopold, K. R.; Canagaratna, M.; Phillips, J. A., Partially bonded molecules from the solid state to the stratosphere. Accts Chem Research 1997, 30, 57-64. 7. Young, N. A., Main group coordination chemistry at low temperatures: A review of matrix isolated Group 12 to Group 18 complexes. Coordination Chem Rev 2013, 257, 956-1010. 8. Phillips, J. A., Structural and energetic properties of nitrile-BX3 complexes: substituent effects and their impact on condensed-phase sensitivity. Theor Chem Accts 2016, 136, 16. 9. Frenking, G.; Wichmann, K.; Frohlich, N.; Loschen, C.; Lein, M.; Frunzke, J.; Rayon, V. M., Towards a rigorously defined quantum chemical analysis of the chemical bond in donoracceptor complexes. Coordination Chem Rev 2003, 238, 55-82.


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10. Swanson, B.; Shriver, D. F.; Ibers, J. A., Nature of the donor-acceptor bond in acetonitrileboron trihalides. The structures of the boron trifluoride and boron trichloride complexes of acetonitrile. Inorganic Chem 1969, 8, 2182-2189. 11. Shriver, D. F.; Swanson, B., Nature of the donor-acceptor interaction in borontrihalide complexes - Vibrational Spectra and vibrational analysis of acetonitrile-boron trichloride and acetonitrile boron tribromide. Inorganic Chem 1971, 10, 1354-1370. 12. Grabowski, S. J., pi-Hole bonds: Boron and aluminum Lewis acid centers. Chem phys chem 2015, 16, 1470-1479. 13. Murray, J. S.; Lane, P.; Clark, T.; Riley, K. E.; Politzer, P., Sigma-Holes, pi-holes and electrostatically-driven interactions. J Molecular Modeling 2012, 18, 541-548. 14. Clark, T., Sigma-holes. Wiley Interdisciplinary Reviews-Computational Molecular Science 2013, 3, 13-20. 15. Donald, K. J.; Tawfik, M., The weak helps the strong: Sigma-holes and the stability of MF4 … base complexes. J Phys Chem A 2013, 117, 14176-14183. 16. Helminiak, H. M.; Knauf, R. R.; Danforth, S. J.; Phillips, J. A., Structural and energetic properties of acetonitrile-group IV (A & B) halide complexes. J Phys Chem A 2014, 118, 4266-4277. 17. Waller, A. W.; Weiss, N. M.; Decato, D. A.; Phillips, J. A., Structural and energetic properties of haloacetonitrile - GeF4 complexes. J Molecular Structure 2017, 1130, 984-993. 18. Eigner, A. A.; Rohde, J. A.; Knutson, C. C.; Phillips, J. A., IR spectrum of CH3CN-BF3 in solid neon: Matrix effects on the structure of a Lewis acid-base complex. J Phys Chem B 2007, 111, 1402-1407. 19. Phillips, J. A.; Cramer, C. J., B-N distance potential of CH3CN-BF3 revisited: Resolving the experiment-theory structure discrepancy and modeling the effects of low-dielectric environments. J Phys Chem B 2007, 111, 1408-1415. 20. Buchberger, A. R.; Danforth, S. J.; Bloomgren, K. M.; Rohde, J. A.; Smith, E. L.; Gardener, C. C. A.; Phillips, J. A., Condensed-phase effects on the structural properties of FCH2CN–BF3 and ClCH2CN–BF3: A matrix-isolation and computational study. J Phys Chem B 2013, 117, 11687–11696. 21. Reeve, S. W.; Burns, W. A.; Lovas, F. J.; Suenram, R. D.; Leopold, K. R., Microwave Spectrum and structure of HCN-BF3 - An almost weakly bound complex. J Phys Chem 1993, 97, 10630-10637. 22. Phillips, J. A.; Halfen, J. A.; Wrass, J. P.; Knutson, C. C.; Cramer, C. J., Large gas-solid structural differences in complexes of haloacetonitriles with boron trifluoride. Inorganic Chem 2006, 45, 722-731.


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Structural and Energetic Properties of HaloacetonitrileâBCl3

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