Matrix Isolation Infrared and DFT Study of the Trimethyl Phosphite

Article pubs.acs.org/JPCA

Matrix Isolation Infrared and DFT Study of the Trimethyl Phosphite− Hydrogen Chloride Interaction: Hydrogen Bonding versus Nucleophilic Substitution N. Ramanathan, Bishnu Prasad Kar, K. Sundararajan, and K. S. Viswanathan*,† Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India S Supporting Information *

ABSTRACT: Trimethyl phosphite (TMPhite) and hydrogen chloride (HCl), when separately codeposited in a N2 matrix, yielded a hydrogen bonded adduct, which was evidenced by shifts in the vibrational frequencies of the TMPhite and HCl submolecules. The structure and energy of the adducts were computed at the B3LYP level using 6-31++G** and aug-ccpVDZ basis sets. While our computations indicated four minima for the TMPhite−HCl adducts, only one adduct was experimentally identified in the matrix at low temperatures, which interestingly was not the structure corresponding to the global minimum, but was the structure corresponding to the first higher energy local minimum. The Onsager self-consistent reaction field model was used to explain this observation. In an attempt to prepare the hydrogen bonded adduct in the gas phase and then trap it in the matrix, TMPhite and HCl were premixed prior to deposition. However, in these experiments, no hydrogen bonded adduct was observed; on the contrary, TMPhite reacted with HCl to yield CH3Cl, following a nucleophilic substitution, a reaction that is apparently frustrated in the matrix. CH3NH−, (CH3)2N−, OCH3−, H−, PH2−, OH−, and F−. With the first seven nucleophiles, they observed an SN2 nucleophilic attack, preferentially at phosphorus, which eventually produced the methoxide anion as a leaving group, while the last three nucleophiles reacted preferentially via a carbon attack to produce a dimethyl phosphite anion as the leaving group. In this paper, we have reported the interaction between TMPhite and HCl, both in a low temperature N2 matrix and in the gas phase. The interaction of TMPhite with HCl in low temperature matrix was found to be different from that of the gas phase. The matrix isolation technique was used to understand the reactivities both in the low temperature matrix and in the gas phase.

1. INTRODUCTION The study of hydrogen bonded adducts, both experimental and theoretical, is of considerable interest.1,2 Studies on hydrogen bonded adducts involving organophosphorus compounds assume significance for various reasons. Organic phosphites and phosphates serve as model systems for understanding biological processes.3,4 Organophosphorous compounds are also used as extractants in a number of solvent extraction processes. Earlier studies from our group reported the hydrogen bonded adducts of trimethyl phosphate with various proton donors such as H2O, C2H2, and C6H6.5−7 The rapid advance of molecular biology owes much to the synthesis of DNA. This synthesis operates by way of phosphites, where the addition of each nucleoside residue is followed by oxidation of the phosphite to phosphate; the resulting phosphates are quite stable under the conditions of the syntheses.8−12 Although there is a vast body of literature dealing with the chemistry of phosphorus compounds, surprisingly only a few studies are reported for lower valence organophosphorous compounds. They were reported to undergo multistep chemical reactions with complicated kinetic analyses.12 In short, the kinetic and mechanistic data are sparse for these lower valence organophosphorous compounds. Anderson et al. reported the first gas phase chemical reaction of trimethyl phosphite (TMPhite).13 They conducted ion− molecule reactions of TMPhite with a variety of nucleophiles such as CH2CHCH2−, (CH3)2CC(CH3)CH2−, NH2−, © 2012 American Chemical Society

2. EXPERIMENTAL SECTION Matrix isolation experiments were carried out using a Leybold AG He-compressor-cooled closed cycle cryostat. The details of the vacuum system and experimental setup are described elsewhere.14−17 TMPhite (98%, Merck) was used without any further purification. However, the sample was subjected to several freeze−pump−thaw cycles before use. HCl gas was prepared by mixing AR grade H2SO4 and HCl solutions, in a Received: July 13, 2012 Revised: November 2, 2012 Published: November 19, 2012 12014

dx.doi.org/10.1021/jp306961m | J. Phys. Chem. A 2012, 116, 12014−12023

The Journal of Physical Chemistry A

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corrections together, as it has been shown that a simultaneous application of both BSSE and ZPE corrections tends to underestimate the binding energies.22 Calculations were also performed using the Onsager selfconsistent reaction field (SCRF) model,23,24 as implemented in the Gaussian program, to determine the effect of the matrix environment on equilibrium geometries and energies of the various TMPhite−HCl adducts. (The motivation for this study will become apparent when the results are discussed.) In this model, the solute molecule is placed in a spherical cavity surrounded by a continuum with constant dielectric properties. The molecular dipole induces a dipole in the dielectric medium and the electric field applied to the solute by the solvent dipole in turn interacts with the molecular dipole, altering the energy of the solute. The radius of the spherical cavity occupied by the solute was estimated based on molecular volumes obtained from the Gaussian calculations.

vacuum bulb attached to a buret. Prior to mixing of the reagents, it was ensured that the vacuum bulb was evacuated to ∼5 × 10−6 mbar. The HCl gas that was produced was trapped in a low temperature bath, which was maintained at a temperature of ∼ −100 °C. DCl gas was prepared by mixing KCl and D2SO4 (96−98 wt % solution in D2O, 99.5 atom %, Aldrich) and freezing out the evolved DCl. TMPhite and HCl were deposited in the matrix by streaming them separately through a twin-jet-nozzle system. In this experiment, TMPhite was mixed with matrix gas, in the required ratio in the mixing chamber, and the resultant mixture was allowed to stream through one nozzle and deposit onto the matrix, with the flow being adjusted by a dosing valve. Through a second nozzle and a dosing valve, HCl gas was deposited. In these experiments, HCl in the reservoir was maintained at a temperature of ∼ −60 °C to control its concentration in the matrix. We also performed experiments, in which TMPhite, HCl (or DCl), and the matrix gas were premixed in a 1-L glass bulb at room temperature (∼298 K). This gas mixture was then streamed out of a single jet nozzle and deposited onto a KBr substrate maintained at 12 K. Deposition was carried out at the rate of ∼3 mmol/h, and a typical deposition lasted for 2 h. The spectra were recorded using a BOMEM MB 100 Fourier transform infrared (FTIR) spectrometer, operated at a spectral resolution of 1 cm−1. After a spectrum was recorded, the matrix was warmed to ∼35 K, maintained at this temperature for 15 min, and recooled to 12 K. The spectra of the matrix thus annealed were again recorded. High purity Ar and N2 (Inox, 99.9995%) were used as matrix gases.

4. RESULTS AND DISCUSSION 4.1. Codeposition of TMPhite and HCl Using a TwinJet Nozzle. 4.1.1. Experimental Procedures. Twin-jet

3. COMPUTATIONAL DETAILS Computations were performed using a Gaussian 94W package,18 running on a Pentium 4 machine with a 2.4 GHz processor. Geometries of the monomers were first optimized at the B3LYP/6-31++G** level of theory. Starting from the optimized monomer geometries, the geometries of the adducts were then optimized. All geometry parameters were left free in the optimization process. Vibrational frequency calculations were then performed to ensure that the computed structures did correspond to minima on the potential surface and also to help us in the assignments of the various vibrational features observed in our experiments. The computed frequencies for the different modes were scaled on a mode-by-mode basis for assigning the experimental features. To arrive at the scaling factor, the strongest feature of the uncomplexed TMPhite observed experimentally was correlated to the strongest feature computed for the ground state conformer of this molecule, in the appropriate region of the spectrum. The factor that would bring this computed frequency into agreement with the experimental feature was calculated; this scaling factor was then used to scale the computed frequencies of the TMPhite adducts. The scaling factors were thus computed for the different regions of the infrared spectrum. We believe that a mode-by-mode scaling is necessary, in order to account for the varying influence the matrix has, on the vibrational frequencies in the different regions of the infrared spectrum.19,20 A few calculations were also performed at the B3LYP/aug-cc-pVDZ level of theory. Stabilization energies were computed for the adducts, which were corrected separately, for zero point energies (ZPE) and basis set superposition errors (BSSE) using the method outlined by Boys and Bernardi.21 We did not apply both

Figure 1. Matrix isolation infrared spectra in a N2 matrix, in the region 2900−2500 cm−1, where TMPhite and HCl were separately codeposited: (a) HCl/N 2 (1.8:1000); (b) HCl/TMPhite/N 2 (1.2:1.0:1000); (c) HCl/TMPhite/N2 (1.8:1.0:1000). Spectra shown here are those recorded at 12 K after annealing the matrix at 32 K. The spectrum of TMPhite alone is not shown, as the molecule shows no absorption in this region. 12015



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Figure 2. Matrix isolation infrared spectra in a N2 matrix, in the regions (A) 1100−980 and (B) 800−720 cm−1, where TMPhite and HCl were separately codeposited: (a) TMPhite/N2 (1.0:1000); (b) TMPhite/HCl/N2 (1.0:1.2:1000); (c) TMPhite/HCl/N2 (1.0:1.8:1000). Spectra shown here are those recorded at 12 K after annealing the matrix at 32 K. The spectrum of HCl alone is not shown, as the molecule shows no absorption in this region.

assignments of these product bands will be discussed in section 4.2. The features of the adduct(s) were clearly resolved from those of the precursor molecules and are marked with arrows in Figures 1−3. All the spectra shown here are those recorded after annealing the matrix. The intensities of the product features increased when the concentration of the either of the two precursors was increased, confirming that these features must be due to TMPhite−HCl adducts. 4.1.2. Computations. The structures of the TMPhite−HCl adducts were computed using 6-31++G** and aug-cc-pVDZ basis sets. Density functional theory (DFT) methodology was adopted through the B3LYP hybrid exchange-correlational functional. In an earlier work,25 the lowest energy conformer of TMPhite was shown to have a C1(TG±G±) structure, with a population of 85%. Three other conformers with Cs(TG+G−), C1(TTG±), and C3(G±G±G±) structures were also found to be minima, with progressively higher energies, and which together

codeposition experiments were performed to characterize the adducts formed between TMPhite and HCl, in a solid N2 matrix. Figures 1 (2900−2500 cm−1) and 2 (1100−980 and 800−720 cm−1) show the infrared spectra obtained in such codeposition experiments. The spectrum shown in Figure 1 corresponds to the H−Cl stretch, while those shown in Figure 2 correspond to the vibrational modes of TMPhite. Figure 3 shows the infrared spectrum, over the region 2100−1800 cm−1, obtained when TMPhite and DCl were codeposited in a N2 matrix; the spectral region spanned corresponds to the DCl stretch. TMPhite−HCl and TMPhite−DCl adducts showed no perceptible changes in the spectral regions corresponding to the TMPhite vibrational modes, and hence, to avoid a repetitive display of spectra, this region has been shown only for the TMPhite−HCl adduct. It can be seen from the above spectra that product bands of the TMPhite−HCl adduct(s) were observed at 2579.6, 1058.9, 1010.8, 1003.9, 785.7, and 744.0 cm−1 in a N2 matrix; 12016



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corrected relative energies are shown in Figure 4. The selected structural parameters of the adducts are shown in Table 1. The complete set of structural parameters in Cartesian coordinates, defining the optimized geometries, of all four adducts of TMPhite−HCl, has been presented in the Supporting Information. Stabilization energies (raw, ZPE corrected, and BSSE corrected) for all the adducts calculated at the B3LYP/6-31+ +G** level are shown in Table 2. Also in Table 2 the stabilization energies of the adducts calculated using the B3LYP/aug-cc-pVDZ level of theory are shown for comparison. It must be pointed out that in adducts I, II and III there is a weak interaction between the Cl atom and one of the methyl hydrogens, i.e., Cl18−H8, Cl18−H11, and Cl18−H16 in the three adducts, respectively. As a result of this interaction, the corresponding C−H bond length is marginally shortened (by 0.05, 0.07, and 0.03%, respectively) and vibrational frequencies are increased (i.e., blue shifted) by about 7−12 cm−1 from their values in free TMPhite. Of course, these are observations from our computational work and we have not been able to unambiguously observe the blue-shifted features in our experiments. 4.2. Vibrational Assignments of TMPhite−HCl Adduct(s). 4.2.1. Features of the HCl Submolecule in the TMPhite−HCl Adduct(s). H−Cl/D−Cl Stretch. Figure 1a shows the matrix isolation infrared spectra of HCl trapped in N2. The feature obtained at 2854.0 cm−1 is assigned to the H−Cl stretch in the monomer.26 The group of features observed at 2852.5, 2842.5, 2815.9, 2800.8, 2792.8, 2778.2, 2766.5, 2760.0, and 2736.0 cm−1 are assigned to the vibrational modes of multimers of HCl.26 The features observed at 2647.0 and 2545.0 cm−1 are assigned for HCl−H2O adducts.27 When HCl was codeposited with TMPhite and the matrix was then annealed, a strong feature was obtained at 2579.6 cm−1 (Figure 1b,c), which is redshifted by ∼275 cm−1 from the HCl monomer feature. The vibrational frequencies together with their assignments are presented in Table 3. Experiments were also performed where TMPhite was codeposited with DCl. In spite of our careful efforts taken during the preparation of DCl, HCl always present as a contaminant. All the features of the DCl monomer, DCl multimers, and HCl−DCl heteromultimers, were observed when DCl was deposited in the N2 matrix.26 When TMPhite was codeposited with DCl, a strong feature was observed at 1881.6 cm−1 (Figure 3), which was assigned to the TMPhite− DCl adduct. The ratio of the 1881.6 and 2579.6 cm−1 features (0.7294), which are respectively the features of the TMPhite− DCl and TMPhite−HCl adducts, is also consistent with this assignment. While our computations predict that there are four different adducts that can possibly be formed between the two precursors, with four different infrared frequencies for the HCl submolecule, we observe only one clear feature in the HCl stretching region. This observation indicates that experimentally only one of the four adducts is formed in the matrix. The question of course is to identify which of the four adducts is formed in the matrix. Adduct IV, where the phosphorus atom is the proton acceptor, is the least stable of the four adducts, with an energy 2.3 kcal/mol higher relative to adduct I, at the B3LYP/6-31+ +G** level. Hence, it is very unlikely that this adduct can be exclusively observed in the matrix and we therefore rule out this adduct from further consideration. Similarly, we can rule out

Figure 3. Matrix isolation infrared spectra in a N2 matrix, in the region 2100−1800 cm −1, where TMPhite and DCl were separately codeposited: (a) DCl/N 2 (1.6:1000); (b) DCl/TMPhite/N 2 (1.6:0.5:1000); (c) DCl/TMPhite/N2 (1.6:1.0:1000). Spectra shown here are those recorded at 12 K after annealing the matrix at 32 K. The spectrum of TMPhite alone is not shown, as the molecule shows no absorption in this region.

had a population of ∼15%. However, owing to the small barriers for conformer interconversion from these higher energy conformers to the ground state conformer, these higher energy conformers were found not to contribute significantly at the low temperature of our experiments. Consequently, only the C1(TG±G±) ground state conformation of TMPhite was considered in this study, on the TMPhite−HCl adducts. Four minima were located on the TMPhite−HCl potential surface, corresponding to four proton acceptor sites on the TMPhite molecule (Figure 4). Adducts I, II, and III represent structures where the alkoxy oxygen on TMPhite served as the proton acceptor, while in adduct IV phosphorus served as the proton acceptor. All these structures have been listed in order of increasing energies. In adducts I and III, the oxygen attached to carbon in the gauche position is the proton acceptor, while in adduct II (the first higher energy local minimum), the oxygen attached to carbon in the trans position is the proton acceptor. Due to the C1 symmetry of the ground state conformer of TMPhite, the three oxygen atoms are nonequivalent, and hence, this results in three possible adducts at oxygen. The structures of the adducts along with the ZPE 12017



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Figure 4. Structures of different TMPhite−HCl adducts. The ZPE corrected energies of the adducts (kcal/mol), relative to that of the global minimum, are given next to each structure.

cm−1 are assigned to the C−O stretch of the ground state C1(TG±G±) conformer of TMPhite.25 When TMPhite and HCl were codeposited and annealed (Figure 2A(b,c)), new spectral features were observed at 1058.9, 1010.8, and 1003.9 cm−1, indicating that these new features are due to the adduct of TMPhite and HCl. Vibrational wavenumbers together with their assignments are presented in Table 3. The features at 1010.8 and 1003.9 cm−1 are likely site split doublets and may therefore be assigned to the same mode. The wavenumbers of adduct I in the C−O stretching region, after appropriate scaling, were computed to occur at 1060.8, 1029.4, and 1021.6 cm−1, while those of adduct II were computed to occur at 1065.5, 1035.6, and 1002.5 cm−1. The computed features near 1030 cm−1 for both adducts are unlikely to be observed in the experiments even if they occur, as they would be masked by the strong features, in this region, of the TMPhite precursor. The other two features near 1060 and 1000 cm−1 lie reasonably removed from the features of monomeric TMPhite and can therefore be observed, if they do occur. The analysis therefore

the possibility of assigning the feature to adduct III, which is present 1.0 kcal/mol above the global minimum. Adducts I and II are placed at relative energies of 0.5 kcal/mol, with adduct I being the lower energy isomer, and are therefore likely contenders for the assignment. Scaled computed frequencies of adducts I and II show red shifts in the stretch of the HCl submolecule of 405 and 304 cm−1, while experimentally we observed a red shift of 275 cm−1 (Table 3). Likewise, the computed shifts for the DCl submolecule in the TMPhite−DCl adduct are 288 and 218 cm−1 for adducts I and II, respectively, while the experimental shift is 185 cm−1. There appears to be, therefore, a better agreement of the experimental and computed wavenumbers for adduct II, which is higher energy by 0.5 kcal/mol, than for adduct I. In other words, we seem to be observing a local minimum rather than the global minimum. 4.2.2. Features of the TMPhite Submolecule in the TMPhite−HCl Adduct. C−O stretch. Figure 2A(a) shows the matrix isolation infrared spectra of TMPhite trapped in N2. The spectral features at 1066.8, 1035.0, 1030.8, 1025.1, and 1022.2 12018



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Table 1. Selected Structural Parametersa of TMPhite−HCl Adducts, Calculated at the B3LYP/6-31++G** Level

found to occur at 782.0, 768.7, and 711.4 cm−1, while those for adduct II were computed to occur at 782.1, 737.0, and 733.7 cm−1. Clearly the two experimentally observed features show a better agreement with the computed features of adduct II than with those of adduct I (Table 3). The computed feature of adduct II at 733.7 cm−1 is not observed in the experiment, as it likely merges with the strong features of TMPhite. While the vibrational wavenumbers for adducts I and II are shown in Table 3, those for adducts III and IV are presented in Supporting Information. The experimental observations together with our computations revealed that the observed adduct features can be assigned to the first higher energy adduct II, rather than to the ground state adduct I. A local minimum being favored over the global minimum is not unusual. Mielke and co-workers reported in their experiments on glyoxal−hydrogen peroxide adducts that the first local minimum structure was observed preferentially in the matrix.28 Similarly, in their experiments on methanol− methyl glyoxal and methanol−diacetyl adducts, they observed only the hydrogen bonded adduct, even though the computations predicted the non-hydrogen-bonded adduct to be the most stable one.29 In another study involving the 1:1 hydrogen bonded adducts between H2O2−HF, H2O2−HCl, and H2O2−HBr in low temperature matrixes, computations predicted two isoenergetic structures: one an open structure and the other a cyclic structure. However, experimentally only the open structure was observed in the Ar and N2 matrixes, which was reported to be stabilized by the interaction with the matrix medium, due to its larger dipole moment.30 A closer examination of our own system revealed that, in the TMPhite−HCl adducts, adduct II (4.49 D) has a higher dipole moment than adduct I (1.90 D). It is therefore likely that the matrix stabilizes preferentially adduct II, which has the larger dipole moment. 4.3. Onsager Solvation Model. To investigate the effects of the matrix on the structure and energetics of the adducts of TMPhite−HCl, the structures of the four adducts were optimized within the Onsager reaction field model. The value of the dielectric constant ε was set equal to 2.0, which was appropriate for the N2 matrix.31−33 Using the Onsager SCRF model, geometry optimizations and frequency calculations were done at the B3LYP/6-31++G** level, for the TMPhite−HCl adducts. It can be seen from Table 4 that, in the N2 matrix, there is an energy reversal of the two adducts. The higher energy adduct II, in the gas phase, becomes the global minimum in the N2 matrix, where ε = 2.0. It turns out that the ZPE corrected energy of adduct I is now higher than that of adduct II by ∼0.2 kcal/mol. This computation lends support to our assignment of the vibrational features to adduct II. The energies for adducts III and IV computed using the Onsager solvation model are also shown in Table 4. While there are changes in energy of these adducts, too, in the matrix, they do not enjoy energy changes that are significant enough to put them in contention. To summarize, the Onsager model indicates adduct II to be the global minimum when the solvent effect of the N2 matrix is taken into account, which lends support to our assignments of the observed vibrational features of the adduct in the N2 matrix to adduct II. 4.4. Results of Experiments Where TMPhite and HCl Were Premixed Prior to Deposition. Since the studies discussed in the previous sections revealed that the matrix plays an important role in the stabilization of the adducts, it was

adducts parameter H17−O2 H17−O3 H17−O4 H17−P1 Cl18−H17 Cl18−H8 Cl18−H10 Cl18−H13 Cl18−H15 Cl18−H16 C5−H8 C6−H11 C7−H16 ∠H17O2P1 ∠H17O3P1 ∠H17O4P1 ∠H17P1O3 ∠Cl18H17O2 ∠Cl18H17O3 ∠Cl18H17O4 ∠Cl18H17P1 tor ∠Cl18H17O2P1 tor ∠Cl18H17O3P1 tor ∠Cl18H17O4P1 tor ∠Cl18H17P1O3 dipole moment (D)

adduct I

adduct II

adduct III

adduct IV

1.8052 1.7672 1.8241 1.3181 3.3873

1.3107

1.3130

2.3920 1.3129 4.3298 3.9809

3.4912 3.1184 1.0941 1.0953 1.0921 125.6 127.8 122.2 120.1 171.5 172.6 173.3 178.5 −175.4 164.5 −159.5 1.90

4.49

4.33

94.5 3.93

a

Bond distances in angstroms; bond angles and torsion angles in degrees. Torsion angles of the fragment ABCD denote the angle between ABC and BCD planes.

Table 2. Stabilization Energies (Raw/ZPE Corrected/BSSE Corrected) of the Different Adducts of TMPhite−HCl, Calculated at the B3LYP/6-31++G** and B3LYP/aug-ccpVDZ Levels of Theory stabilization energy (kcal/mol) adducts adduct adduct adduct adduct

I II III IV

B3LYP/6-31++G**

B3LYP/aug-cc-pVDZ

−6.77/−5.33/−6.21 −6.24/−4.79/−5.52 −5.85/−4.33/−5.39 −4.23/−2.99/−3.96

−5.83/−4.35/−5.84 −5.24/−3.87/−5.12 −4.90/−3.49/−4.97 −3.73/−2.55/−3.50

concentrates on these two features. The root-mean-square deviation of these two experimental features from those of the computed features is smaller for adduct II than for adduct I, which indicates that the adduct trapped in the matrix is probably adduct II, a conclusion that is consistent with that arrived at, based on the analysis of the H−Cl vibration. P−O stretch. Figure 2B shows the spectrum, in the region of the P−O stretch, obtained when TMPhite and HCl were codeposited in the N2 matrix. The spectral features at 780.6, 775.8, 760.1, 742.2, 736.7, and 731.4 cm−1 are assigned to the P−O stretch of the ground state C1(TG±G±) conformer of TMPhite.25 When TMPhite and HCl were codeposited and annealed at 32 K (Figure 2B( b,c), new spectral features were produced at 785.7 and 744.0 cm−1 that were assigned to the TMPhite−HCl adduct. The computed frequencies of adduct I in the P−O stretching region, after appropriate scaling, were 12019



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Table 3. Computed (Unscaled and Scaled) and Experimental Wavenumbers, Scaling Factors, and Assignments for TMPhite− HCl Adducts I and II, in the N2 Matrixa vibrational wavenumbers (cm−1) unscaled computedb ν (cm−1) 2952.7 (21) 2536.0 (1129) 2637.9 (1057) 2124.8 (11) 1828.9 (566) 1900.8 (522) 1037.4 (326); 1053.3 (307); 1081.6 (88) 1033.9 (300); 1041.8 (338); 1073.6 (75) 1014.5 (292); 1048.1 (311); 1078.3 (128) 697.7 (179); 737.1 (150); 749.7 (79) 690.0 (192); 745.6 (121); 758.5 (94) 711.6 (152); 714.9 (246); 758.6 (89)

scaling factor

scaled computedc ν (cm−1) 2854.0 2451.0 (−403.0) 2549.5 (−304.5) 2066.5 1778.8 (−287.7) 1848.7 (−217.8) 1025.1, 1041.2, 1068.7

0.9666

0.9726

0.9881

1021.6 (−3.5); 1029.4 (−11.8); 1060.8 (−7.9) 1002.5 (−22.6); 1035.6 (−5.6); 1065.5 (−3.2) 719.3, 760.1, 772.9

1.031

711.4 (−7.9); 768.7 (+8.6); 782.0 (+9.1) 733.7 (+14.4); 737.0 (−23.1); 782.1 (+9.2)

experimentalc ν (cm−1)

mode assignment

2854.0 − 2579.6 (−274.4) 2066.5 − 1881.6 (−184.9) 1022.2, 1025.1; 1030.8, 1035.0; 1066.8

H−Cl stretch of HCl monomer H−Cl stretch of adduct I H−Cl stretch of adduct II D−Cl stretch of DCl monomer D−Cl stretch of adduct I D−Cl stretch of adduct II C−O stretch of TMPhite

−

C−O stretch of adduct I

1003.9 (−19.6); 1010.8 (−12.7); −; 1058.9 (−7.9) 731.4, 736.7, 742.2; 760.1; 775.8, 780.6

C−O stretch of adduct II P−O stretch of TMPhite

−

P−O stretch of adduct I

−; 744.0 (−16.1); 785.7 (+7.5)

P−O stretch of adduct II

a

The computations were performed at the B3LYP/6-31++G** level. Data for the monomeric molecules are shown in italics. bIntensties, in km/mol, given in parentheses. cShift in vibrational wavenumber of the adduct with respect to the monomer bands, (νadduct − νmonomer), given in parentheses.

Table 4. Influence of Dielectric Constant on the Energies of TMPhite−HCl Adducts, Calculated at B3LYP/6-31++G** Level of Theory, Using the Onsager Solvation Model relative energies of adducts (kcal/mol) adduct I

adduct II

adduct III

adduct IV

dielectric const

raw

ZPE corr

raw

ZPE corr

raw

ZPE corr

raw

ZPE corr

0.000 (gas) 2.000 (N2)

0.000 0.000

0.000 0.000

0.530 −0.230

0.538 −0.188

0.921 0.277

1.001 0.324

2.542 2.666

2.342 2.550

HCl was used (TMPhite:HCl was 1:4), only features due to HCl and CH3Cl were observed. The gas phase premixing therefore produced no hydrogen bonded adduct; on the contrary, TMPhite readily reacted with HCl to form CH3Cl (Figure 6). The other product of the reaction, dimethyl phosphite, which tautomerizes to dimethyl hydrogen phosphonate (DMHP),36 was not clearly observed in the matrix, as the PO containing molecules, such as phosphates and phosphonates, adsorb on metal surfaces such as the copper deposition lines and are therefore inefficiently transported to the matrix. Furthermore, when excess of HCl was used, it is also possible for the DMHP to hydrolyze further to yield monomethyl hydrogen phosphonate and phosphorous acid, which have low vapor pressures to be transported to the matrix. Far from providing evidence of the hydrogen bonded adduct I, the gas phase premixing experiments revealed a new reaction channel, a nucleophilic substitution, between TMPhite and HCl, in the gas phase. Obviously, this substitution reaction is frustrated in the matrix, due, possibly to the cage effect, where the hydrogen bonded adduct is the only product formed. Reactions of organic phosphites with alkyl halides or aryl halides is well-known in the literature as the Arbuzov reaction.37−42 In this reaction, the trialkyl phosphite, P(OR1)3 reacts at elevated temperatures, with an alkyl halide, R2X, to yield the dialkyl phosphonate, OP(R2)(OR1)2, and the alkyl halide, R1X. The reaction proceeds through a nucleophilic

thought interesting to prepare the adduct in the gas phase, where there is no matrix medium to alter the energies of the adducts, and then deposit the adduct in the matrix. Such a method would likely reveal that adduct I, which is the lower energy isomer in the gas phase, is the one likely to form and which, once deposited in the matrix, may be preserved due to the cage effect. When TMPhite and HCl were premixed with Ar in the ratio 1:1:1000, and the matrix was then deposited, we observed intense product bands at 3040.8, 2965.1, 2866.9, 1445.5, 1349.4, 1015.0, 722.4, and 716.7 cm−1 in Ar matrix. In N2 matrix, these product bands occurred at 3048.2, 2968.3, 2870.2, 1445.4, 1353.4, 1026.8, 732.6, and 727.3 cm−1 (Figure 5). When the matrix was annealed at ∼35 K, no perceptible changes in the spectra were observed. These spectral features are clearly very different from those obtained when TMPhite and HCl were separately codeposited through a twin-jet nozzle. The intense product bands listed above agree well with the reported features of CH3Cl in an Ar matrix.34,35 It is therefore clear that in, the gas phase, TMPhite and HCl react to produce CH3Cl. Table 5 lists all the observed frequencies in both Ar and N2 matrixes. The intensity of the CH3Cl features in the matrix increased as the concentration of HCl was increased in the premixed gas mixture. When an excess of TMPhite was used, such as in the experiments where the TMPhite:HCl ratio was 2:1, features due to TMPhite and CH3Cl were observed. When an excess of 12020



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Figure 5. Matrix isolation infrared spectra in a N2 matrix, in the regions (A) 3100−2700, (B) 1500−1300, (C) 1100−1000, and (D) 800−700 cm−1, where TMPhite and HCl were premixed prior to deposition: (a) TMPhite/N2 (1:1000); (b) HCl/N2 (1.8:1000); (c) TMPhite + HCl mixture/N2 (1:1:1000). Spectra shown here are those recorded at 12 K. The feature marked with an asterisk is due to CH3OH.

directs the kinetic product, while when the reaction is frustrated in the matrix cage, the stronger base leads to the formation of the thermodynamically stable product. In addition to the intense product bands of CH3Cl, weak bands of CH3OH were observed in our gas phase mixing experiments. The CH3OH feature in the N2 matrix at 1034.8 cm−1 is marked with an asterisk in Figure 5. The observation of traces of CH3OH in our experiments could be due to the reaction of TMPhite with water, an inevitable impurity in any matrix isolation experiment. In fact, we observed traces of CH3OH in our experiments, even when no HCl was added, which strongly suggests that this is a product due to the reaction of TMPhite with water. To check if this reaction occurs in the liquid phase, we added hydrochloric acid to TMPhite, which is a liquid, and the vapor phase above this mixture was then swept with a stream of N2

attack by the phosphite with the electrophilic alkyl halide, to yield the above-mentioned products. In this work, we have reported for the first time the reaction of TMPhite with HCl in the gas phase at room temperature (unlike the higher temperatures reported for the alkyl halides), with a mechanism similar to that proposed for the reactions with alkyl halides (see Figure 6). It may be pointed out that the Arbuzov reaction is initiated by a nucleophilic attack by the lone pair of electrons on phosphorus, as this center is more nucleophilic than the alkoxy oxygen. However, hydrogen bonding is more favored at the alkoxy oxygen owing to the stronger basicity of the alkoxy oxygen relative to the phosphorus. This results in oxygen centered adducts I, II, and III to be more stable than the phosphorus centered adduct IV. It can therefore be concluded that, in the gas phase, where the Arbuzov reaction is not frustrated, the stronger nucleophile 12021



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products using a B3LYP/6-31++G** level of theory. For TMPhite, the most stable C1(TG±G±) geometry was taken into account. Similarly, only the ground state conformation of product, DMHP (G±G∓), was considered.43 The calculated free energy change for this substitution reaction was found out to be ∼ −10 kcal/mol, which indicates the feasibility of the reaction at room temperature.

Table 5. Vibrational Wavenumbers of the CH3Cl Product Bands Trapped in Ar and N2 Matrixesa vibrational wavenumber (cm−1) Ar matrix

N2 matrix

symmetry

mode assignment

2965.1

2968.3

A1

ν1 (νsCH3)b

1349.4

1353.4

A1

ν2 (δsCH3)b

722.4 716.7 3040.8

732.6 727.3 3048.2

E

ν3 (νsC35Cl)b ν3 (νsC37Cl)b ν4 (νaCH3)b

1445.5

1445.4

E

ν5 (δdCH3)b

1015.0

1026.8

E

ν6 (γCH3)b

2866.9

2870.2

A1

5. CONCLUSIONS Matrix isolation infrared spectroscopy was used to study the interaction between TMPhite and HCl both in the low temperature matrix and in the gas phase. When TMPhite and HCl were separately codeposited in a N2 matrix, hydrogen bonded adduct was produced which was corroborated using DFT computations. Computations using the B3LYP/6-31+ +G** level of theory indicated four types of adducts, but only one adduct (adduct II), whose dipole moment was the maximum, was observed in the low temperature matrix. Interestingly, this adduct did not correspond to the lowest energy adduct, but to a first higher energy adduct. However, Onsager self-consistent reaction field model calculations indicated that this higher energy adduct (adduct II) is strongly stabilized in a N2 matrix and in fact turns out to be the global minimum structure when matrix perturbations are taken into account. In the gas phase, TMPhite and HCl did not produce the hydrogen bonded adduct but resulted in a nucleophilic substitution reaction, in which the major product was CH3Cl.

ν2 + ν5c

a

CH3Cl is produced by the gas phase reaction between TMPhite and HCl. bAssignment from ref 34. cAssignment from ref 35.

and deposited onto the cold KBr substrate. Vibrational features of CH3Cl and CH3OH were found in the matrix, with the features due to CH3OH being dominant, probably due to the hydrolysis of TMPhite by aqueous hydrochloric acid. The presence of features due to CH3Cl, though weak, however indicates that the nucleophilic substitution reaction does occur in the solution phase, too. 4.4.1. Computations for the CH3Cl Reaction Channel. To rationalize the reaction between TMPhite and HCl in the gas phase, we performed free energy calculations for this reaction. Free energy values were calculated for both the reactants and

Figure 6. Scheme showing the reaction between TMPhite and HCl in the gas phase to produce dimethyl hydrogen phosphonate (DMHP) and CH3Cl, through an Arbuzov type of reaction. 12022



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ASSOCIATED CONTENT

S Supporting Information *

Structural parameters of optimized geometries, in Cartesian coordinates, of the various TMPhite−HCl adducts computed at the B3LYP level of theory using 6-31++G** basis set and computed (unsacled and scaled) and experimental wavenumbers, scaling factors, and assignments for the TMPhite− HCl adducts III and IV, in the N2 matrix. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Department of Chemistry, Indian Institute of Science Education & Research, Sector 81, Mohali 140306, Punjab, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.P.K. gratefully acknowledges the grant of a research fellowship from IGCAR, Department of Atomic Energy, India. REFERENCES

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Matrix Isolation Infrared and DFT Study of the Trimethyl Phosphite

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