Experimental and Theoretical Study of Molecular Response of Amine

Article pubs.acs.org/JPCB

Experimental and Theoretical Study of Molecular Response of Amine Bases in Organic Solvents Shawn M. Kathmann,*,† Herman Cho,† Tsun-Mei Chang,‡ Gregory K. Schenter,† Kshitij Parab,† and Tom Autrey† †

Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Department of Chemistry, University of WisconsinParkside, Kenosha, Wisconsin 53141, United States

‡

ABSTRACT: Reorientational correlation times of various amine bases (namely, pyridine, 2,6-lutidine, 2,2,6,6-tetramethylpiperidine) and organic solvents (dichloromethane, toluene) were determined by solution-state NMR relaxation time measurements and compared with predictions from molecular dynamics (MD) simulations. The amine bases are reagents in complex reactions catalyzed by frustrated Lewis pairs (FLP), which display remarkable activity in metal-free H2 scission. The comparison of measured and simulated correlation times is a key test of the ability of recent MD and quantum electronic structure calculations to elucidate the mechanism of FLP activity. Correlation times were found to be in the range of 1.4−3.4 (NMR) and 1.23−5.28 ps (MD) for the amines and 0.9−2.3 (NMR) and 0.2−1.7 ps (MD) for the solvent molecules.

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methods. The recent findings of Pu and Privalov21 that the dynamical reactive pathway for H2 activation is fundamentally different than that deduced via the minimum-energy path illustrates that explicit treatment of molecular dynamics is essential for deducing reaction mechanisms in these systems. Mixtures of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane (BCF) exhibit the ability of FLPs to activate hydrogen,2−11 but while other reactive amine bases interact strongly with BCF, often forming stable dative complexes in solution, little experimental evidence of acid−base interactions has been found for the TMP/BCF pair.11 Intermediates of the hydrogen-splitting reaction have likewise eluded direct observation. Sumerin and co-workers observed the formation of a paleyellow color upon mixing TMP with BCF in benzene accompanied by a downfield shift of the amine protons. They suggested that this could be indicative of a hydrogen-bonding interaction between the protic hydrogen on the amine with fluorine groups on BCF. A violet-colored complex formed upon mixing trimesitylphosphine with BCF has been observed and was proposed to arise from π-stacking of the electron-poor borane with the electron-rich phosphine arene rings.22 Hypothetical mechanisms for the hydrogen-splitting reactions of FLPs with amine bases have been proposed on the basis of density functional theory (DFT) electronic structure calculations.7,8,17,23,24 These studies model H2 scission as the product of a thermally induced encounter of the acid, base, and H2 in specific spatial configurations. A molecular dynamics (MD) study of the trajectories leading to transient encounters of reagents has recently been reported by Dang et al.16 They

INTRODUCTION Frustrated Lewis pairs (FLPs) display remarkable catalytic ability to activate small molecules.1 We are particularly interested in FLP activation of H2 and catalysis of the reduction of polar substrates at ambient temperature and pressure. The acid and base of a FLP do not form a conventional dative adduct either for steric reasons, in the case of intermolecular pairs, or due to a rigid bonding framework in the case of intramolecular pairs.2−11 Gas-phase calculations indicate that the acid and base can associate via weak van der Waals forces,12−14 but in condensed phases, the chemically relevant interactions are more difficult to discern and appear to be rather weak.15,16 Proposed mechanisms for H2 activation in solution include a termolecular reaction of dissolved hydrogen gas randomly diffusing toward a noninteracting Lewis acid and base in a solvent cage and a bimolecular reaction of dissolved H2 diffusing into a solvent cage where the FLPs have some weak or strong association. However, experimental evidence of key intermediates and weakly interacting FLP encounter complexes in solution has been inconclusive, and recent investigations into reaction mechanisms have relied heavily on molecular simulation methods. Although the formation of a dative bond is “frustrated” in such systems, gas-phase calculations17−20 find interaction energies between the Lewis acid and base in the range of 5−15 kcal/mol, whereas molecular dynamics15,16 modeling of encounter complexes in solution have revealed interactions along the potential of mean force on the order of kBT. The purpose of the present study is to evaluate the ability of classical molecular dynamics to accurately model diffusive orientational motions for Lewis bases. In conjunction with explicit electronic structure calculations, a credible molecular dynamics approach can elucidate mechanisms of FLP activity that have proven problematic to probe by experimental © 2014 American Chemical Society

Received: January 23, 2014 Revised: March 27, 2014 Published: April 2, 2014 4883

dx.doi.org/10.1021/jp500821u | J. Phys. Chem. B 2014, 118, 4883−4888

The Journal of Physical Chemistry B

Article

nuclear electric quadrupole moment via torques, which, in turn, cause transitions between the nuclear spin states, leading to the re-establishment of an equilibrium Boltzmann distribution. Similarly, correlation times of solvent molecules can be extracted from T1 of the quadrupolar 2H isotope on labeled organic solvent molecules. As seen above, reorientational correlation times can be obtained from both NMR measurements and MD simulations, for amine bases and organic solvents alike. Determinations of the correlation times thus present a crucial quantitative benchmark of the ability of combining ab initio data and classical MD simulations to describe the dynamics of Lewis bases in solution, particularly transient species that may be difficult to observe by spectroscopic means.

obtained correlation functions that, while not directly observable, may be used to predict relaxation times in liquids by well-established methods.25,26 The Debye−Stokes−Einstein (DSE) model expresses the exponential decay of the lth rank orientational correlation function due to rotational relaxation as Cl(t ) = e−t/ τl

(1)

where the reorientational correlation time is τl = 6ηV/l(l + 1) kBT and V is the volume of the molecule (obtained via the liquid mass density). The DSE model assumes a hydrodynamic stick boundary condition for isotropic rotational diffusion of a molecular sphere in a viscous fluid. For l = 2, it follows that τ2,DSE = ηV/kBT, and the reorientational correlation time is determined from the decay of the autocorrelation function C2(t ) = ⟨P2[u(0) ·u(t )]⟩

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EXPERIMENTAL AND THEORETICAL METHODS NMR Spectroscopy. Nitrogen-14 measurements were performed on an Agilent Inova 500 MHz instrument (ν(14N) = 36.107 MHz; T = 25 °C) equipped with a Nalorac 5 mm HXY probe, and 2H experiments were carried out on a Tecmag Discovery 300 MHz spectrometer (ν(2H) = 46.078 MHz; T = 25 °C). The lock channel of a Nalorac 10 mm HX probe was used to observe the 2H signal. Although this method of 2H detection precluded field locking of the magnet, the field drift, measured at −4.0 × 10−4 ppm/h, had negligible effect on the observed line widths over the duration of the experiment. Relaxation curves for both nuclides were measured by the inversion recovery method with CYCLOPS phase cycling of the 90° pulse and receiver phases. Relaxation times were extracted by fits of these curves to a single exponential decay function in accordance with the DSE rotational relaxation model. Correlation coefficients for all fits were ≥0.991, with no sign of a systematic deviation from the fitting function. All samples were prepared and stored under a nitrogen atmosphere prior to all NMR measurements. Computations. Quadrupole moments used in calculating correlation times with eq 3 were the values Q = 0.00286 × 10−24 (2H) and 0.0208 × 10−24 cm2 (14N) recommended by Raghavan.31 The EFG parameters Vzz and ηQ were obtained using the code QTRANS.f32 with the results from Gaussian9833 with optimized structures; in addition, the results for nitrogen sites were computed at the B3LYP/6-31++G** level of theory and basis set. The classical molecular dynamics were simulated with Amber934 using nonpolarizable general Amber force field (GAFF) parameters for all species studied. Periodic boundary conditions were imposed, and long-range Coulomb interactions were treated with the Ewald summation, and a MD time step of 2 fs was used. The partial atomic charges were calculated from an ab initio RESP-fit at the HF/6-311+G* level and basis set also using Gaussian98. The molecular dynamics simulations were done in three stages using appropriate statistical mechanical ensembles (1 ns/stage): (1) constant number of molecules (N = 276 solvent molecules + 1 base molecule =277), constant pressure (P = 1 atm), and constant temperature (T = 300 K) to ensure the correct mass density of the system (i.e., the NPT ensemble); (2) constant number of molecules, constant volume, and constant temperature using the resulting average volume from stage 1 (i.e., the NVT ensemble); and (3) constant number of molecules, constant volume, and constant energy (i.e., the NVE ensemble) to gather the exact dynamics used in the evaluation of C2(t) as defined in eq 2. For example, from the NPT simulations, we obtain a TOL mass density of 0.852 g/cm3, which agrees well with the experimental mass density of 0.867 g/cm3.35

(2)

where u(t) is a time-dependent molecule-fixed unit vector at time t, P2(x) = (3 cos2 x − 1)/2 is the second-order Legendre polynomial, and ⟨...⟩ denotes an ensemble average. The rotation of nonspherical molecules on the other hand can be decomposed into rotations about three molecular-based axes u(j), as illustrated in Figure 1, each with its own reorientational correlation time τ(j) l .

Figure 1. Molecule-fixed unit vectors used to calculate the reorientational correlation times for PYR. One vector (u(1)) is parallel to the line between carbon atoms on the ring adjacent to the nitrogen atom, and a second vector (u(2)) points along the line between the nitrogen atom and the carbon atom across the ring. The third axis (u(3)) is the cross product of u(1) and u(2).

The reorientational correlation time τc of the amine bases can be related to NMR parameters through the expression27−30 T1−1 =

≈

2 3 2I + 3 ⎛⎜ eQ ⎞⎟ 2 40 I (2I − 1) ⎝ ℏ ⎠

∫0

∞

⟨Vzz(0)Vzz(t )⟩ dt

⎛ η 2 ⎞⎤ 2⎡ 3 2I + 3 ⎛ eVzzQ ⎞ ⎢ Q ⎟⎥ ⎜ ⎟ 1 + ⎜ ⎜ 3 ⎟⎥τ2 40 I 2(2I − 1) ⎝ ℏ ⎠ ⎢⎣ ⎝ ⎠⎦

(3)

14

where T1 is the N spin−lattice relaxation time in the extreme narrowing limit, I is the nuclear spin quantum number, Vzz is the zz component of the electric field gradient (EFG) tensor in its principal axis system, ηQ is the EFG tensor asymmetry parameter, and Q is the nuclear quadrupole moment (in eq 3, it is assumed that Vzz(t) = Vzz(0)e−t/τ2). The microscopic mechanism28 of quadrupole relaxation can be considered to occur via random thermal motion of the solvent, causing fluctuations in the EFG at the nuclear site that couples with the 4884



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from the fit exponentials are presented in Table 2. The motions of LUT and TMP display significant rotational anisotropy, as indicated by a distribution of correlation times and functions, whereas the three correlation functions for PYR are

RESULTS AND DISCUSSION Amine Bases. Calculated and measured 14N quadrupole coupling data for 2,6-lutidine (LUT), pyridine (PYR), and TMP appear in Table 1. Experimental PYR results have been Table 1. Nitrogen-14 Quadrupolar Data for Amine Bases molecule

|eQVzz/h| (MHz)

ηQ

methoda

PYR

4.903 4.584 4.908 4.603 4.572 5.632

0.33 0.40 0.42 0.18 0.30 0.04

QMb NQRc MWd QMb MWe QMb

LUT TMP a

QM = quantum mechanical calculation; MW = microwave spectroscopy; NQR = nuclear quadrupole resonance spectroscopy. b This work. cRubenacker and Brown.36 dHeineking et al.37 eThomsen and Dreizler.38

obtained in both condensed36 and gas37 phases, whereas for LUT, only gas-phase measurements are available.38 To our knowledge, quadrupole couplings have not been reported for TMP. Nitrogen-14 T1 times for the bases dissolved to concentrations of ∼0.10 M in toluene are shown in Table 2. Reducing Table 2. Comparison of Reorientational Correlation Times for Amine Bases in Toluene Obtained by 14N NMR, MD Simulations, and DSE Theory τ2 (ps) MD molecule PYR LUT TMP

14

N T1 (ms) 1.91 0.93 0.70

(1)

NMR/QM

u (t)

u(2)(t)

u(3)(t)

DSE

1.4 3.4 3.0

1.23 2.65 5.28

1.23 4.12 4.45

1.31 3.32 3.41

17.8 25.6 37.5

the base concentrations by 50% resulted in negligible changes in T1. Relaxation times derived from the relation Δ = (πT1)−1, where Δ is the full width half-height line width, were in close agreement with the T1 results from inversion recovery methods. Reorientational autocorrelation times in the third column of Table 2 were computed following eq 3, using T1 values from the second column and Vzz data from QM calculations (Table 1). Relaxation time measurements were also performed on a sample prepared by the addition of 1 equiv of BCF to the 0.10 M TMP solution. The presence of BCF had no distinguishable effect on the T1 or chemical shift (−294.0 ppm) of 14N on TMP, suggesting the absence of a conventional Lewis acid−base interaction between the two molecules. These observations are consistent with recent experiments on chemical shifts39 as well as recent simulation studies that found very weak interactions (about −0.5 kcal/mol) between BCF and LUT in toluene, as measured by the potential of mean force.16 A perturbation in the 1H NMR chemical shift of the amine protons of TMP when BCF is added to solution has been previously reported, but 19 F NMR spectra displayed no corresponding change that would indicate hydrogen bonding between the TMP NH and a BCF fluorine. Plots of simulated C2(t) functions are displayed in Figure 2 along with fits of the simulation points to a single exponential function, in accordance with eq 1. Correlation times extracted

Figure 2. Reorientational autocorrelation functions C2(t) of u(1)(t), u(2)(t), and u(3)(t). The plots shown are for PYR (top), LUT (middle), and TMP (bottom), all in a toluene solution. The solid line is a fit of a single exponential function to the data for the indicated axis u(j)(t).

substantially equivalent. The influence of methyl group substitution in lowering both the rate and symmetry of molecular rotations can be seen in the LUT and PYR results; while correlation times for rotations about the in-plane axes u(1)(t) and u(2)(t) are found to be equal for PYR, for LUT, the correlation time for u(2)(t) is 55% larger than that for u(1)(t). The experimental measurements and dynamics simulations both find correlation times for LUT and TMP that are more 4885



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than double τ2 for PYR. The best quantitative agreement with experiment is seen for τ(3) 2 , with the largest absolute error equal to 13.7% in the case of TMP. Every axis shows the same progression of increasing correlation times in the direction TMP > LUT > PYR. Kauzmann40 has proposed that reorientation may be viewed as proceeding by jump diffusion over free-energy barriers separating more stable solute−solvent configurations. The inverse of the relaxation time is equated to the jump frequency, which allows Transition-State Theory to be used to estimate the rotational free-energy barriers of the Lewis bases in toluene via ΔG‡ = kBT ln[kBTτ2/h] from the experimental relaxation times to yield ΔG‡(PYR) = 1.29 kcal/mol, ΔG‡(LUT) = 1.82 kcal/mol, and ΔG‡(TMP) = 1.75 kcal/mol. These reorientational barriers have been found to follow the barriers from viscosity measurements given by ΔE‡ = kBT ln[η/ηo].41 For example, from viscosity data, activation barriers for pure TOL42 and pure PYR43 may be estimated as ΔE‡(TOL) = 2.10 kcal/mol and ΔE‡(PYR) = 4.04 kcal/mol. Additional studies will investigate the activation energies as determined from the temperature dependence of the reorientational correlation times, which, however, is beyond the scope of the current work. Correlation times obtained with the DSE theory (Table 2, seventh column) exhibit the same trends as the experimental and dynamics results but with magnitudes up to 15 times greater. It is important to note that the calculated DSE relaxation time was based on a continuum stick boundary condition valid when the solute molecule is much larger than the solvent molecules.44 However, when solute and solvent molecules are similar in size, the stick boundary condition breaks down, and the solute starts to slip against the surrounding solvent molecules. This is usually expressed as τ2,DSE = ηVC/kBT, where C is called the “coupling parameter” and takes on the values C = 1 (stick) and C < 1 (slip). In general, 0 < C < 1, and if we use the experimental relaxation times and calculate C for the three bases, we obtain C(PYR) = 78.65 × 10−3, C(LUT) = 132.81 × 10−3, and C(TMP) = 80.00 × 10−3. Some authors45,46 following the stick boundary condition find the hydrodynamic volume that best fits the experimental reorientational times. For the present cases, this would result in hydrodynamic volumes that are unreasonably small compared to their bulk liquid volumes at ambient conditions (in parentheses), V(PYR)= 10.54 Å 3 (133.77 Å 3 ), V(LUT) = 25.59 Å3 (192.32 Å3), and V(TMP) = 22.58 Å3 (282.60 Å3). The DSE model further predicts that the first- and secondorder relaxation times differ by a factor of 3, that is, τ1 ≈ 3τ2, where τ1 is the characteristic decay time of the function ⟨P1[u(0) · u(t)]⟩ and P1(x) = x is the first-order Legendre polynomial. The ratios obtained from the simulation correlation times (Table 3) are seen to lie within 30% of the

Solvent Molecules. The 2H T1 times for CD3C6D5 (TOL-d8) and CD2Cl2 (DCM-d2) measured in this work (Table 4) are in close agreement with results obtained by Glasel at 31 °C using adiabatic fast passage methods (1.08 (CH3C6D5) and 3.18 s (CD2Cl2)).48 As with the 14N data, relaxation times determined by inversion recovery were consistent with values calculated from the full width at half-height line width. The experimental EFG data in Table 4 are from 2H NMR measurements on frozen Table 4. Deuterium EFG, T1, and τ2 Data of Dichloromethane and Toluene τ2 (ps) molecule DCM-d2 TOL-d8: para meta ortho methyl a

(1) τ(1) 1 /τ2

(2) τ(2) 1 /τ2

(3) τ(3) 1 /τ2

PYR LUT TMP

2.58 3.58 2.10

2.81 3.54 2.18

2.97 2.62 2.85

a

160.2 192.5b 179.9c 203.2b 179.9c 202.5b 179.9c 201.3b 193.8b

ηQ

2 a

0.00 0.05b 0.06c 0.07b 0.06c 0.07b 0.06c 0.08b 0.03c

H T1 (s)

NMR

MD

2.88

0.9 0.6 3.0 2.3 2.2 1.7 2.2 1.8

0.2

0.70 0.96 0.94

1.7 1.4 1.4

4.67

Kunwar et al.49 bThis work. cBarnes and Bloom.50

DCM49 and TOL.50 As seen in the table, the 2H quadrupole couplings obtained by gas-phase quantum mechanical calculations are systematically larger than values measured in the solid state. Reorientational correlation times τ2 computed with the T1, Vzz, and ηQ as inputs to eq 3 appear in the fifth column of Table 4; τ2 from the simulation fits are shown in the last column. These data reveal that correlation times obtained from experimental measurements are consistently larger than τ2 values inferred from the simulations and moreover that τ2,TOL > τ2,DCM. The magnitudes of the TOL hydrogen correlation times display the same order ortho ≈ meta < para in both simulations and experiments. In view of the 2H NMR spectra observed by Barnes and Bloom, which exhibited no measurable difference in the quadrupolar splittings of the TOL ring deuterons, it is unlikely that site variations of Vzz account for the anomalous para τ2. A more plausible explanation is that anisotropic motion of the phenyl ring at these temperatures leads to trajectories with different effective reorientation rates for the para deuteron. One such trajectory is the rotation of the ring about the axis collinear with the para C−D bond, which reorients the C−D bonds at the ortho and meta positions but not at the para site. A similar explanation has been proposed for the differences in correlation times determined from 2H T1 measurements of the different hydrogen sites in formamide.51 Again, we find reasonable agreement between the experimental and simulated results for both DCM and TOL, with all simulated results being consistently slightly smaller than experiment. Both experiment and simulation find smaller reorientational correlation times for DCM compared to TOL. We note that the calculated quadrupole coupling constants are quite sensitive to the molecular geometries, for example, in DCM eQVzz/h = 192.5 kHz for a C−D bond length of 1.0869 Å, whereas eQVzz/h = 149.2 kHz for a C−D bond length of 1.1269 Å. Table 5 compares the reorientational correlation times for TOL with the various bases. For all cases, we find τ(1) 2 ≈ (2) τ(3) 2 > τ2 .

Table 3. Comparison of τ1/τ2 Ratios of Amine Bases in Toluene from MD Simulations molecule

eQVzz/h (kHz)

DSE prediction, showing that, as anticipated, intermolecular correlations between the solvent molecules and base molecules are quite weak.47 Any further analysis of the deviations from the DSE predictions is beyond the scope of the current work. 4886



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Table 5. Comparison of Toluene τ2 Values in Solutions with Amine Bases from MD Simulations

Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

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τ2 (ps)

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solute

u(1)

u(2)

u(3)

PYR LUT TMP

2.96 2.24 2.31

1.76 1.53 1.69

2.75 2.64 2.61

(1) Stephan, D. Frustrated Lewis Pairs I: Uncovering and Understanding; Springer-Verlag: Berlin, Germany, 2013; p 1. (2) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124−1126. (3) Herrington, T. J.; Thom, A. J. W.; White, A. J. P.; Ashley, A. E. Dalton Trans. 2012, 41, 9019−9022. (4) Stephan, D. W. Dalton Trans. 2012, 41, 9015. (5) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476− 3477. (6) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050−8053. (7) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskelä, M.; Repo, T.; Pyykkö, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117−14119. (8) Sumerin, V.; Schulz, F.; Nieger, M.; Leskelä, M.; Repo, T.; Rieger, B. Angew. Chem., Int. Ed. 2008, 47, 6001−6003. (9) Erő s, G.; Mehdi, H.; Pápai, I.; Rokob, T. A.; Király, P.; Tárkányi, G.; Soós, T. Angew. Chem. 2010, 122, 6709−6713. (10) Bertini, F.; Lyaskovskyy, V.; Timmer, B. J.; de Kanter, F. J.; Lutz, M.; Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K. J. Am. Chem. Soc. 2011, 134, 201−204. (11) Karkamkar, A.; Parab, K.; Camaioni, D. M.; Neiner, D.; Cho, H.; Nielsen, T. K.; Autrey, T. Dalton Trans. 2013, 42, 615−619. (12) Rokob, T.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. Angew. Chem., Int. Ed. 2008, 47, 2435−2438. (13) Hamza, A.; Stirling, A.; Rokob, T.; Pápai, I. Int. J. Quantum Chem. 2009, 109, 2416−2425. (14) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 1402−1405. (15) Bako, I.; Stirling, A.; Balint, S.; Pápai, I. Dalton Trans. 2012, 41, 9023. (16) Dang, L. X.; Schenter, G. K.; Chang, T.-M.; Kathmann, S. M.; Autrey, T. J. Phys. Chem. Lett. 2012, 3, 3312−3319. (17) Wu, D.; Jia, D.; Liu, L.; Zhang, L.; Guo, J. J. Phys. Chem. A 2010, 114, 11738−11745. (18) Pyykkö, P.; Wang, C. Phys. Chem. Chem. Phys. 2010, 12, 149− 155. (19) Gao, S.; Wu, W.; Yirong, M. Int. J. Quantum Chem. 2011, 111, 3761−3775. (20) Zeonjuk, L.; Vankova, N.; Mavrandonakis, A.; Heine, T.; Röschenthaler, G.-V.; Eicher, J. Chem.Eur. J. 2013, 19, 17413− 17424. (21) Pu, M.; Privalov, T. J. Chem. Phys. 2013, 138, 154305. (22) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880−1881. (23) Erös, G.; Nagy, K.; Mehdi, H.; Pápai, I.; Nagy, P.; Király, P.; Tárkányi, G.; Soós, T. Chem.Eur. J. 2012, 18, 574−585. (24) Camaioni, D. M.; Ginovska-Pangovska, B.; Schenter, G. K.; Kathmann, S. M.; Autrey, T. J. Phys. Chem. A 2012, 116, 7228−7237. (25) Debye, P. J. W. Polar Molecules; Dover: New York, 1929; Vol. 172. (26) Hansen, J.; McDonald, I. Theory of Simple Fluids, 2nd ed.; Academic: New York, 1986. (27) Abragam, A. Principles of Nuclear Magnetism; Clarendon: Oxford, U.K., 1961. (28) Hynes, J. T.; Wolynes, P. G. J. Chem. Phys. 1981, 75, 395. (29) Badu, S.; Truflandier, L.; Autschbach, J. J. Chem. Theory Comput. 2013, 9, 4074. (30) Odelius, M. Thesis: Molecular Dynamics Simulations of Intermolecular Mechanisms in Nuclear Spin Relaxation; Stockholm, Sweden, 1994; www.fysik.su.se/∼odelius/PDF/PhDthesis.pdf. (31) Raghavan, P. Atom. Data Nucl. Data Tables 1989, 42, 189−291. (32) Bailey, W. Calculation of Nuclear Quadrupole Coupling Constants in Gaseous State Molecules. http://nqcc.wcbailey.net/index.html (2005).

CONCLUSION The magnitudes and trends of the correlation times measured by NMR relaxation experiments are found to be in close agreement with the correlation times obtained from single exponential fits to the reorientational correlation functions for both the amine base and the solvent. Furthermore, in the cases studied here, the simple DSE rotational model qualitatively reproduces the trends in the absolute reorientational correlation times of the amine bases, and the predicted DSE proportionality τ1 ≈ 3τ2 is reasonably obeyed when compared to simulations and experiment. However, our analysis shows that DSE slip rather than stick boundary conditions are more appropriate to reproduce both the measured and simulated reorientational times. Furthermore, we find that the rotational barriers from rate theory are small (∼1.6 kcal/mol as given by the average of the three barriers for the bases in toluene) and slightly below the viscous barrier for pure toluene. These results imply that the coupling interactions between the solute and solvent molecules are weak. The 14N NMR measurements presented here, like previous experimental reports, revealed no evidence of a TMP/BCF adduct, transient or otherwise. In particular, 14N chemical shifts and relaxation times of the amine group in TMP were found to be unchanged by the addition of BCF. This finding is supported by MD simulations and quantum electronic structure calculations, which find only weak interactions and short-lived encounters between the acid and base, as was also found in our previous simulation study on potentials of mean force for the FLP LUT/BCF in TOL.16 The absence of a loosely bound acid− base pair in solution is also consistent with MD simulations reported for PR3 and BCF in toluene by Bako et al.15 and Rocchigiani et al.39 and suggests that hydrogen activation by an intermolecular FLP is a fundamentally termolecular reaction. If correct, an optimal configuration for hydrogenation catalysis may be the linked intramolecular FLPs. Work is in progress to investigate this hypothesis. Given that FLP reaction intermediates are difficult to detect by spectroscopic methods, we anticpate that validated MD simulations, like those employed in the present study, will play an important role in elucidating and optimizing FLP catalytic activity.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work benefited from discussions with Don Camaioni, Mark Bowden, Abhi Karkamkar, and Bojana GinovskaPangovska. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific 4887



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