Evidence for Phosphorus Bonding in Phosphorus Trichloride

Article pubs.acs.org/JPCA

Evidence for Phosphorus Bonding in Phosphorus Trichloride− Methanol Adduct: A Matrix Isolation Infrared and ab Initio Computational Study Prasad Ramesh Joshi, N. Ramanathan, K. Sundararajan, and K. Sankaran* Chemistry Group, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, Tamil Nadu, India S Supporting Information *

ABSTRACT: The weak interaction between PCl3 and CH3OH was investigated using matrix isolation infrared spectroscopy and ab initio computations. In a nitrogen matrix at low temperature, the noncovalent adduct was generated and characterized using Fourier transform infrared spectroscopy. Computations were performed at B3LYP/6-311++G(d,p), B3LYP/aug-cc-pVDZ, and MP2/6-311+ +G(d,p) levels of theory to optimize the possible geometries of PCl3−CH3OH adducts. Computations revealed two minima on the potential energy surface, of which, the global minimum is stabilized by a noncovalent P···O interaction, known as a pnictogen bonding (phosphorus bonding or P-bonding). The local minimum corresponded to a cyclic adduct, stabilized by the conventional hydrogen bonding (Cl···H−O and Cl···H−C interactions). Experimentally, 1:1 P-bonded PCl3− CH3OH adduct in nitrogen matrix was identified, where shifts in the P−Cl modes of PCl3, O−C, and O−H modes of CH3OH submolecules were observed. The observed vibrational frequencies of the P-bonded adduct in a nitrogen matrix agreed well with the computed frequencies. Furthermore, computations also predicted that the P-bonded adduct is stronger than H-bonded adduct by ∼1.56 kcal/mol. Atoms in molecules and natural bond orbital analyses were performed to understand the nature of interactions and effect of charge transfer interaction on the stability of the adducts.

1. INTRODUCTION Studies on noncovalent interactions are a century old and have greatly been reinforced due to their enormous importance.1 These forces play a vital role in understanding the properties of liquids and three-dimensional structures adapted by DNA, proteins, and crystal solids. In this context, hydrogen bonding (H-bonding) interaction has been studied in great detail using both experimental and theoretical methods owing to its significance in chemical and biological research.2 Conventional H-bonding is commonly represented by a notation A−H···D (where A stands for electron acceptor and D stands for electron donor). The delocalization of electron density from atom D to antibonding orbital of A−H results in lengthening of A−H bond and hence shift in its stretching vibrations to lower frequencies.3−15 In addition to the conventional H-bonding, studies on unconventional H-bonding such as O−H···π and N−H···π were also reported,16−19 where π electrons acted as an electron donor. A new class of H-bonding interactions, namely proton shared/transferred H-bonding, were the subject of various studies in the literature.20−28 In contrast to normal Hbonding, recently unusual improper H-bonding has also been reported especially in C−H bond of CHCl3, CHF3, and C6H6.29−36 Another class of noncovalent interaction that has been extensively studied is the halogen bonding in which the intervening position of the hydrogen atom is replaced by a halogen atom, X.37−45 This interaction involves direct attractive force between two electronegative atoms due to its highly © 2015 American Chemical Society

anisotropic electrostatic potential. In addition to H or X bond interactions, although not thoroughly studied, other noncovalent interactions involving O and S atoms have been reported, and they are referred as chalcogen bonding.46−49 Recent computational studies have revealed a new facet of noncovalent intermolecular interaction, recognized as pnictogen (the term is also referred as pnicogen or pnigogen)50 bonding wherein the atom of the Lewis acid involved in the interaction is any one of the pnictogen atoms (N, P, and As).51 It was first discerned by Solimannejad et al. during the investigation of blue-shifted H-bonds when HSN was paired with either aliphatic amines or phosphine (PH3). In their study of HSN with PH3, rather than the expected P−H···N interaction, interestingly, a new type of H−P···N interaction was found to be more stable.52 Scheiner further examined the interaction between PH3 and NH3 and reported that H−P···N interaction was twice more stable than the P−H···N interaction.53 Effects of substitution, by replacing one of the H atoms of PH3 by electron withdrawing groups or carbon chain on the strength of pnictogen bonding have been examined.54−56 Furthermore, computations revealed that there is no enhancement on the stability of pnictogen bond when more than one hydrogen atom of PH3 is replaced by Received: November 7, 2014 Revised: March 13, 2015 Published: March 14, 2015 3440

DOI: 10.1021/jp511156d J. Phys. Chem. A 2015, 119, 3440−3451

The Journal of Physical Chemistry A

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using aug-cc-pVDZ and 6-311++G(d,p) basis sets without imposing any geometrical constraints and then the geometry of the PCl3−CH3OH adducts. Interaction energies were computed for the adducts, corrected separately, for basis set superposition errors (BSSE) using the method outlined by Boys and Bernardi78 and zero-point energies. Turi and Dannenberg showed that simultaneous applications of both BSSE and ZPE corrections tend to underestimate the interaction energies; thus, both these corrections were not applied together.79 The optimized geometries were then used to obtain the vibrational frequencies which enable us to characterize the nature of stationary points and to assign the experimentally obtained frequencies. Indeed, all the structures discussed in this work correspond to minima on the potential energy surface. The computed intensities and scaled frequencies were used before plotting simulated a vibrational spectrum using SYNSPEC program,80 by assuming with a Lorentzian line profile a full-width-at-half-maximum of 1 cm−1. Atoms in molecules (AIM) theory proposed by Bader was used to understand the nature of the interaction between the PCl3 and CH3OH.81 A (3, −1) bond critical point (BCP) that could be associated with the intramolecular interaction within monomer and with the intermolecular interaction between the two submolecules was estimated. The electron density ρ(rc), Laplacian of electron density ∇2ρ(rc), and the ratio of the eigenvalues |λ1|/λ3 were examined at BCP to better understand the nature of the interaction. Weak interactions are characterized by small values of ρ(rc) and ∇2ρ(rc) > 0.82 Identification and analysis of the critical points were done using AIM2000 package.83 Natural bond orbital (NBO; version 3.1) analysis invoked through Gaussian G94W was used to understand the nature of hyper conjugative charge−transfer interactions in PCl3−CH3OH adducts.84 Onsager self-consistent reaction field (SCRF) model,85−87 as implemented in the Gaussian program, was used to demonstrate the effect of the matrix environment on equilibrium geometries and energies of both PCl3−CH3OH adducts.

electronegative atoms or groups. Pnictogen bonds other than the P···N interaction have also been reported, showing the trend of strength as P···N > P···O > P···S > P···π.57,58 Del Bene et al. studied the structure, binding energy, spin−spin coupling constant and NMR properties of P···P, P···N, and P···Cl pnictogen bonds, and the effect of substituents on these interaction.59−62 Detailed comparison of the pnictogen bond with hydrogen and halogen bond interactions was studied by various groups.63−69 Recent computations have revealed that vibrational spectroscopy in the far-infrared region and depolarized Raman scattering techniques could be a promising tool for investigating the pnictogen bonding.70 Although, there is extensive research carried out on the pnictogen bonding from the computational point of view, the experimental reports on this type of interaction are sparse. X− ray crystallography was used to study the P···N interaction in aminoalkylferrocenyldichlorophosphanes, P···P interactions in pentafluorophenyl substituted diphosphine, aminotetra phospines, and As···As interactions in cyclopentadienyl arsenic compounds.71−74 Nuclear magnetic resonance (NMR) technique has been used for the study of P···P type of pnictogen interactions in molecules having trivalent phosphorus atom.75,76 In the present work, we report the noncovalent P···O (pnictogen) interaction between PCl3 and CH3OH studied using matrix isolation infrared spectroscopy. The experimental results are supported with the aid of computational studies.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS The prerequisite of a low temperature for matrix isolation experiments were achieved using a RDX-408D2 (Sumitomo Heavy Industries Ltd.) pulsed tube, closed cycle helium compressor cooled cryostat. A base pressure of less than 1 × 10−6 mbar was obtained in the cryostat housed in an evacuated vacuum chamber. Analytical grade PCl3 (Merck, purity: >99%) and HPLC grade CH3OH (purity: >99%) were used without any further purification. However, the samples were subjected to several freeze−pump−thaw cycles before use. Nitrogen (Inox) with a purity of 99.9995% was used as the matrix gas. PCl3 and CH3OH were deposited onto a KBr substrate maintained at 12 K by streaming them separately through a twin-jet-nozzle system. PCl3 was mixed with nitrogen in a mixing chamber and the resultant mixture was allowed to stream through one nozzle and deposited onto the matrix, with the flow being adjusted by a dosing valve. A second nozzle was utilized for the deposition of CH3OH. Here a bulb containing CH3OH was maintained at different temperatures ranging from −80 to −100 °C to control the required dynamic vapor pressure and thereby the concentrations in the matrix. Typical sample to matrix ratio ranging from 1 to 3:1000 for PCl3:N2 and 1 to 3:1000 for CH3OH:N2 were used. A typical deposition lasted for about 75 min at a rate of ∼3 mmol/h. Infrared spectra of matrix-isolated samples were recorded in transmission mode between 4000 and 400 cm−1 using a BOMEM MB100 FTIR spectrometer with 1 cm−1 resolution. After deposition, the matrix was slowly warmed to 30 K, maintained at this temperature for about 15 min, and then recooled to 12 K. The spectra of the matrix thus annealed were again recorded. All the spectra reported refer to the samples annealed at 30 K unless otherwise specified. Ab intio calculations were performed on the PCl3−CH3OH system using a Gaussian 94W package.77 Geometries of the submolecules were first optimized at B3LYP and MP2 levels

3. RESULTS AND DISCUSSION 3.1. Experimental Section. Figure 1a shows the infrared spectrum corresponding to P−Cl stretching vibrational modes of PCl3, spanning the region 520−470 cm−1. The doubly degenerate P−Cl stretching mode (ν3) of PCl3 was reported to occur at 498.9 and 494.9 cm−1 in an argon matrix.88 The ν3 vibrational mode of PCl3 in a nitrogen matrix is observed at 496.6, 494.2, 492.3, 491.1, 502.1, and 499.5 cm−1, as shown in Figure 1a. Since chlorine has two isotopes of mass numbers 35 and 37 amu with a natural abundance of 75.77:24.23, there is a possibility of different isotopic ratios of 35Cl/37Cl (3/0, 2/1, 1/ 2, and 0/3 atoms) present in PCl3, which would result in six different vibrational frequencies for the above mode (two different frequencies for each of the 2/1 and 1/2 isotopic combinations and two more frequencies originated from 3/0 and 0/3 combinations). The probability of occurrence of isotopic combination for two Cl atoms has been estimated previously for 1,2-dichloropropane.89 Similar methodology was applied for three Cl atoms in PCl3 and the natural abundance for the 35Cl/37Cl isotopic 3/0, 2/1, 1/2, and 0/3 combinations are estimated as 43.50%, 41.73%, 13.35%, and 1.42%, respectively. Furthermore, degeneracy of P−Cl stretch may get split due to perturbation by the host matrix atom. Hence, these multiple signatures observed in this region in a nitrogen 3441



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Figure 2. Spectra of the PCl3−CH3OH adduct in nitrogen matrix spanning the regions 3670−3640 cm−1 (grid A) and 1060−1020 cm−1 (grid B); matrix isolation infrared spectra for various concentrations of PCl3/CH3OH/N2. (a) 0/1/1000; (b) 1/1/1000; (c) 2/1/1000; and (d) 3/1/1000. All spectra are recorded at 12 K after annealing at 30 K.

Figure 1. Spectra of the PCl3−CH3OH adduct in a nitrogen matrix spanning the region 520−470 cm−1; matrix isolation spectra for various concentrations of PCl3/CH3OH/N2. (a) 1/0/1000; (b) 1/1/ 1000; (c) 1/2/1000; and (d) 1/3/1000. All spectra are recorded at 12 K after annealing at 30 K.

and form the adducts. The new features observed gained in intensity as the concentration of the precursors were increased, thus indicating that these spectral features are only due to PCl3 and CH3OH adducts. Further, these features were observed at very low concentrations of PCl3 and CH3OH and could be attributed to 1:1 PCl3−CH3OH adducts. 3.2. Computational Section. In order to substantiate our experimental findings, ab initio computations were performed for the PCl3−CH3OH adducts at the B3LYP and MP2 levels of theory using B3LYP/aug-cc-pVDZ and 6-311++G(d,p) basis sets. Figure 3 shows two possible structures for the PCl3− CH3OH adducts computed at the B3LYP/aug-cc-pVDZ level of theory. Interestingly, adduct A, wherein the interaction between P of PCl3 and O of CH3OH was found to be the global minimum, is referred to as pnictogen-bonding (phosphorus bonding or P-bonding). A cyclic adduct B fits the classical description of traditional H-bonding between H of CH3OH and Cl of PCl3 was the local minimum. The structural parameters for adducts A and B at different levels of theory are presented in Table 1. The bond distance P7···O5 ranges between 2.791 and 2.972 Å in the case of adduct A and the bond distance H6···Cl9 between 2.671 and 2.824 Å in the case of adduct B at different levels of theory. The local minimum adduct B is stabilized by ∠Cl9H6O5 (angle) ranging between 155 and 170°, whereas in the case of adduct A, ∠Cl9P7O5 is more linear, ranging between 175 and 179°. Indeed, the above observations are in good agreement with the computational study carried out by Scheiner on the PH3−NH3 system wherein the P···N interaction was reported to dominate over the Hbonding interaction.53 Raw, ZPE, and BSSE corrected interaction energies for adducts A and B at different levels of theory are listed in Table 2. As can be seen from the table at all levels of theory and at different basis sets, adduct A is more strongly bound than adduct B. Although there are deviations in the numerical values

matrix may either be due to isotopic effect of chlorine/splitting of degenerate vibrational states/matrix site effect or a combination of two or more of these effects. Features observed at 502.1 and 499.5 cm−1 could probably be due to PCl3 selfaggregate, which will be discussed in the later section. When concentrations of PCl3 and CH3OH were varied and codeposited with nitrogen and then annealed, new features appeared at 481.7 and 476.7 cm−1, as shown in Figure 1b−d. The features observed at 511.1, 509.2, and 507.2 cm−1 are due to three isotopic combinations of nondegenerate P−Cl stretching frequency ν1(a1) mode of PCl3. Figure 2a shows the infrared spectrum in the regions 3670− 3640 cm−1 (grid A) and 1060−1020 cm−1 (grid B), where O− H and O−C stretching vibrational modes of CH3OH appear. The O−H stretch of CH3OH occurs at 3663.5 cm−1 in a nitrogen matrix.90−92 In the codeposition experiment of PCl3 and CH3OH, a new feature appeared at 3650.0 cm−1, as shown in Figure 2b−d (grid A). The feature observed at 1034.0 cm−1 corresponds to the O−C stretch of monomeric CH3OH.93 Multiple features at 1050.9, 1047.9, 1039.9, 1037.1, 1032.3, and 1028.7 cm−1 were observed when CH3OH:N2 alone was deposited, shown in Figure 2a (grid B). Features at 1050.9, 1039.9, 1032.3, and 1028.7 cm−1 are due to CH3OH dimer, features at 1041.5 and 1037.1 cm−1 are assigned for CH3OH multimer, and the feature at 1047.9 cm−1 is due to the CH3OH−H2O adduct.90,93,94 Water is an inevitable impurity and possibly forms the CH3OH−H2O adduct and water aggregates. Figure 2b−d, grid B, shows new features at 1029.9 and 1026.6 cm−1 in the codeposition experiments of PCl3 and CH3OH. The above new features were observed only when both PCl3 and CH3OH were codeposited and annealed. It is well-known that during annealing the precursors PCl3 and CH3OH diffuse 3442



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3.3.1. P−Cl Doubly Degenerate Stretching ν3 (e) Region. The P−Cl stretching mode of adduct A was computed to occur at 458.6 and 455.6 cm−1, whereas for the adduct B at 471.8 and 464.3 cm−1 for 35Cl:37Cl = 3:0. Furthermore, computation showed that for the isotopic combination of 35Cl:37Cl = 2:1 this mode was found to occur at 456.8 and 454.5 cm−1. The occurrence of doublet is a direct consequence of splitting of the degenerate vibrational mode as a result of complexation. In the case of adduct A, this mode is red-shifted by 11.9, 14.9 cm−1 and 16.0 and 10.0 cm−1 for 35Cl:37Cl = 3:0 and 35Cl:37Cl = 2:1, respectively, whereas for the adduct B, it is blue- and red-shifted by 1.3 and 6.2 cm−1 with respect to monomeric PCl3. Experimentally, new peaks were observed at 481.7 and 476.7 cm−1 in this region, a red shift of 14.9 cm−1 and 15.6 cm−1, when PCl3 and CH3OH were codeposited. This agrees well with the computed shifts for the isotopic combinations of 3:0 and 2:1 of adduct A, respectively, stabilized by P-bonding. Figure 4 compares the matrix isolation spectra recorded for PCl3:N2 (1 to 3:1000) alone and PCl3:CH3OH:N2 (1:3:1000) codeposition experiments with computed spectra of PCl3 monomer (all possible isotopic combinations are considered) and PCl3 dimer. As it is evident from Figure 4, features at 481.7 and 476.7 cm−1 appear only in the codeposition experiments of PCl3 and CH3OH in nitrogen matrix. This observation clearly supports that the new features are assigned to the 1:1 adduct and not due to PCl3 aggregates. No new features could be discerned for the nondegenerate ν1(a1) P−Cl stretching mode of PCl3. 3.3.2. O−C Stretching Mode of CH3OH. Computation shows that the O−C stretching vibrational mode occurs at 1037.1 and 1045.5 cm−1 for adducts A and B, respectively (Table 3). As can be seen from the table, adduct A is redshifted by 4.6 cm−1 and adduct B is blue-shifted by 3.8 cm−1 with respect to the monomeric CH3OH feature. Experimentally, new features were observed as a doublet at 1029.9 and 1026.6 cm−1, with a red shift of 4.1 and 7.4 cm−1, which agrees well with the computed feature of 4.6 cm−1 of adduct A. The

Figure 3. Computed structure of PCl3−CH3OH adducts at the B3LYP/aug-cc-PVDZ level of theory. The figure shows the bond critical points (BCP) and ring critical point (RCP) for both the adducts A and B.

of energies with the different basis sets, the trends in the energies are found to be qualitatively similar for these adducts. 3.3. Vibrational Assignments. Table 3 shows the experimental and computed vibrational frequencies at the B3LYP/aug-cc-pVDZ level of theory.

Table 1. Selected Structural Parameters of Adducts A and B Calculated at B3LYP/aug-cc-pVDZ, B3LYP/6-311++G(d,p), and MP2/6-311++G(d,p) Levels of Theory level of theory B3LYP/aug-cc-pVDZ parameters

a

P7−O5 P7−Cl9 O5−H6 O5−C1 H6−Cl9 H4−Cl8 ∠H6O5P7 ∠Cl9P7O5 ∠C1O5P7 ∠Cl9H6O5 ∠H6Cl9P7 tor∠H6O5P7Cl8 tor∠C1O5P7Cl8 tor∠O5H6Cl9P7 tor∠C1O5H6Cl9 dipole moment

adduct A 2.972a 2.118 (2.104)d 0.965 (0.964) 1.431 (1.427)

adduct B 2.112 (2.104) 0.965 (0.964) 1.426 (1.427) 2.824 4.084

100.4b 175.0 124.9

B3LYP/6-311++G(d,p) adduct A 2.863 2.114 (2.095) 0.962 (0.961) 1.429 (1.424)

2.104 (2.095) 0.962 (0.961) 1.423 (1.424) 2.797 3.762

110.9 175.0 124.9 167.9 112.6

−10.6 −132.8b

2.46c

adduct B

adduct A 2.791 2.077 (2.059) 0.961 (0.959) 1.426 (1.422)

170.1 118.1

3.23

adduct B 2.069 (2.059) 0.960 (0.959) 1.422 (1.422) 2.671 3.581

116.3 179.0 112.4

−13.1 −147.6 175.2 92.9 2.59

MP2/6-311++G(d,p)

156.2 106.7 −43.5 −168.2

−136.4 92.9 3.05

3.04

−78.7 23.5 2.96

Bond length in Ǻ . bBond and torsion angles in deg. cDipole moment in Debye dWhere relevant, the parameters for the monomers are given in parentheses.

a

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Table 2. Rawa/ZPE-Corrected/BSSE-Corrected Interaction Energies (kcal/mol) for Adducts A and B Computed at B3LYP/ aug-cc-pVDZ, B3LYP/6-311++G(d,p), and MP2/6-311++G(d,p) Levels of Theory interaction energy (kcal/mol)

a

adduct

B3LYP/aug-cc-pVDZ

B3LYP/6-311++G(d,p)

MP2/6-311++G(d,p)

adduct A adduct B ΔEb

−2.64/−1.94/−2.25 −0.69/−0.38/−0.50 1.95/1.56/1.75

−3.83/−3.07/−3.01 −1.07/−0.69/−0.38 2.76/2.38/2.63

−7.15/−6.34/−2.95 −3.95/−3.26/−0.75 3.20/3.08/2.20

Interaction energies obtained without corrections either ZPE or BSSE. bAdduct BInteractionEnergy − adduct AInteractionEnergy.

Table 3. Computed and Experimental Vibrational Frequencies, Shifts in the Frequencies and Mode Assignments for the Adducts A and Ba unscaled computed vibrational frequencies ν (cm−1) 3825.5 3819.9 3658.0 3812.7 3811.5 1041.7 1064.5 1032.6 1037.1

(30)c (41) (525) (32) (100) (115) (85) (136) (129)

1045.5 (94) 490.8 489.1 487.1 484.7 470.5 470.5 466.8 465.5 468.5 465.4 491.0 489.1 472.8 471.3 466.9 482.8 458.6 455.6 490.6 471.8 464.3 480.5 454.5 456.8

(25) (26) (27) (24) (145) (145) (141) (142) (142) (142) (23) (23) (250) (132) (130) (40) (169) (169) (35) (145) (143) (40) (165) (122)

experimental vibrational frequencies (in N2 matrix) ν (cm−1)

Δνb

−5.6 −167.5 −12.8 −14.0 22.8 −9.7 −4.6 3.8

−1.7 −3.7 −6.1

−3.7 −5.0 −2.0 −5.1 0.2 −1.7 2.3 0.8 −3.6 −8.0 −11.9 −14.9 −0.2 1.3 −6.2 −10.3 −16.0 −10.0

Δνb

CH3OH Region 3663.5 3655.2, 3651.2 3650.0 d 1034.0 1050.9 1032.3, 1028.7 1029.9 1026.6 d PCl3 Region 511.1 509.2 507.2 d 496.6 (vs) 492.3 (s) 494.2 490.8 (sh) 494.2 d

502.1 499.5 494.2 d 481.7

−8.3, −12.3 −13.5

16.9 −1.7, −5.3 −4.1 −7.4

vibrational mode assignments ν(O−H) in CH3OH ν(O−H) in (CH3OH)2 ν(O−H) in adduct A ν(O−H) in adduct B ν(C-O) in CH3OH ν(C-O) in (CH3OH)2 ν(C-O) in adduct A ν(C-O) in adduct B

−1.9 −3.9

−4.3 −2.4 −5.8 −2.4

ν1 (P−Cl, a1)e in PCl3 (35Cl:37Cl = 3:0) ν1 (P−Cl, a1) in PCl3 (35Cl:37Cl = 2:1) ν1 (P−Cl, a1) in PCl3 (35Cl:37Cl = 1:2) ν1 (P−Cl, a1) in PCl3 (35Cl:37Cl = 0:3) ν3(P−Cl, e)f in PCl3 (35Cl:37Cl = 3:0) ν3 (P−Cl) in PCl3 (35Cl:37Cl = 2:1) ν3 (P−Cl) in PCl3 (35Cl:37Cl = 1:2) ν3(P−Cl, e) in PCl3 (35Cl:37Cl = 0:3) ν1 (P−Cl, a1)e in (PCl3)2 (35Cl:37Cl = 3:0)

5.5 2.9 −2.4 −14.9

ν3(P−Cl, e)f in (PCl3)2 (35Cl:37Cl = 3:0)

ν1 (P−Cl) in adduct-A (35Cl:37Cl = 3:0) ν3(P−Cl) in adduct-A (35Cl:37Cl = 3:0)

d d

ν1 (P−Cl) in adduct-B (35Cl:37Cl = 3:0) ν3 (P−Cl) in adduct-B (35Cl:37Cl = 3:0)

d 476.7

ν1 (P−Cl) in adduct-A (35Cl:37Cl = 2:1) ν3 (P−Cl) in adduct-A (35Cl:37Cl = 2:1)

−15.6

a Computations were performed using the B3LYP/aug-cc-pVDZ level of theory. bShift = Δν = νadduct − νmonomer. cIntensities in km/mol given in parentheses. dFeatures are not observed experimentally. eP−Cl nondegenerate vibrational stretching mode (ν1). fP−Cl doubly degenerate vibrational stretching mode (ν3).

feature observed at 1026.6 cm−1 could be due to the matrix site effect. Additionally, Figure 5 (grid B) compares the matrix isolation spectra recorded for CH3OH:N2 (1 to 3:1000) alone and PCl3:CH3OH:N2 (3:1:1000) codeposition experiments with computed spectra of CH3OH monomer and CH3OH dimer. As it is evident from Figure 5 (grid B), assigned features appear only during PCl3 and CH3OH codeposition experiment

and are absent when CH3OH alone was deposited in the N2 matrix. These observations clearly unveiled that the new features observed in the codeposition experiments are due only to the P-bonded adduct. 3.3.3. O−H Stretching Mode of CH3OH. Computations revealed that the O−H stretching modes of the adducts A and B occur at 3812.7 and 3811.5 cm−1, respectively, a red shift of 3444



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computed shift matches well with the experimental shifts for both adducts A and B, we assign the experimental feature to adduct A. This was based on the agreement between the computed and experimental shifts in the other modes of PCl3 and CH3OH of adduct A. Furthermore, the energetics also favor the production of adduct A at low temperature matrix, which may preclude the assignment of the observed experimental features to adduct B. Figure 5 (grid A) compares the matrix isolation spectra recorded for CH 3 OH:N 2 (1 to 3:1000) alone and PCl3:CH3OH:N2 (3:1:1000) codeposition experiments with computed spectra of CH3OH monomer and CH3OH dimer. The features at 3655.2 and 3651.2 cm−1 have already been assigned for the CH3OH dimer.95 It is apparent from Figure 5 (grid A) that the features of CH3OH aggregates differ from the new feature of our codeposition experiments, which evidently unfolds the exclusive observation of the P-bonded adduct. When the effect of the N2 matrix was considered by taking into account its dielectric constant in the Onsager model calculation, the calculation did not reveal any perceivable change in the relative energy of the adducts A and B. Therefore, it can be surmised that owing to comparable dipole moments of adducts A and B, the extent of stabilization of both the adducts in the N2 matrix is similar in magnitude. 3.3.4. PCl3 Self-Aggregate. Figure 6 shows the structure of the PCl3 dimer computed at the B3LYP/aug-cc-pVDZ level of

Figure 4. Comparison of the experimental and computed spectra spanning the region 520−470 cm−1: computed spectrum of (a) PCl3 monomer*; (b) PCl3 dimer*; matrix isolation infrared spectrum of PCl3/CH3OH/N2 (c) (1/0/1000); (d) (2/0/1000); (e) PCl3/ CH3OH/N2 (3/0/1000); and (f) PCl3/CH3OH/N2 (1/3/1000). All matrix isolation infrared spectra are recorded at 12 K after annealing at 30 K. *Computed frequencies are scaled before plotting (scaling factors: ν1 P−Cl stretching mode = 1.0414 and ν3 P−Cl stretching mode = 1.0555). The computed spectra are plotted assuming Lorentzian line profile with the line width of 1 cm−1.

Figure 6. Computed structure of the PCl3 dimer and the CH3OH dimer at the B3LYP/aug-cc-PVDZ level of theory.

theory. The P−Cl stretching mode for the isotopic combination 35Cl:37Cl = 3:0 of the PCl3 dimer was computed to occur at 472.8, 471.3, and 466.9 cm−1 which are blue-shifted by 2.3 and 0.8 cm−1 and red-shifted by 3.6 cm−1 with respect to the monomer feature (470.5 cm−1), respectively. Experimentally, features observed at 502.1, 499.5, and 494.2 cm−1 (blue shift by 5.5 and 3.9 cm−1 and a red shift by 2.4 cm−1) agree well with the computed shifts of the PCl3 dimer. Thus, these features could be tentatively assigned for the PCl3 dimer. Based on the computational vibrational shifts, the feature at 494.2 cm−1 can also be assigned for isotopic combinations of the PCl3 monomer (35Cl:37Cl = 2:1 and 1:2). However, due to the closeness of vibrational shifts, there is intricacy in the assignment of this feature. Importantly, it should be pointed out that the features of the PCl3−CH3OH adduct are different from the PCl3 aggregate. A systematic and detailed study is required to unambiguously confirm that these features are indeed due to self-aggregate of PCl3.

Figure 5. Comparison of the experimental and computed spectra spanning the regions 3670−3640 cm−1 (grid A) and 1060−1020 cm−1 (grid B): Computed spectrum (a) CH3OH monomer*; (b) CH3OH dimer*; matrix isolation infrared spectrum of PCl3/CH3OH/N2 (c) (0/1/1000); (d) (0/2/1000); (e) (0/3/1000); and (f) (3/1/1000). All matrix isolation infrared spectra are recorded at 12 K after annealing at 30 K. *Computed frequencies are scaled before plotting (scaling factors: νO‑H stretching mode = 0.9576 and νC‑O stretching mode = 0.9926). The computed spectra are plotted assuming Lorentzian line profile with the line width of 1 cm−1.

12.8 and 14.0 cm−1 relative to monomeric O−H stretching mode of CH3OH. Experimentally, we observed a new feature at 3650.0 cm−1, a red shift of 13.5 cm−1. Even though the 3445



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Table 4. Properties of (3, −1) Bond Critical Points for the Adducts A and B Computed at the B3LYP/aug-cc-pVDZ Level of Theory adducts

ρ(rc)a

adduct A adduct Bd adduct Be

0.01356 0.00621 0.00088

PCl3 adduct A adduct B

0.11712 0.11479 0.11558

CH3OH adduct A adduct B

0.36288 0.36128 0.36168

CH3OH adduct A adduct B

0.24752 0.24380 0.24847

∇2ρ(rc)b

λ1c

λ2c

(a) Intermolecular Bond Critical Points in PCl3−CH3OH Adducts 0.03804 −0.01037 −0.00962 0.01710 −0.00524 −0.00501 0.00261 −0.00045 −0.00030 (b) P−Cl Bond of PCl3 −0.11164 −0.12149 −0.11226 −0.11235 −0.12046 −0.11236 −0.11558 −0.11988 −0.11118 (c) O−H Bond of CH3OH −2.04471 −1.84602 −1.80325 −2.06244 −1.85189 −1.81103 −2.04929 −1.84720 −1.8049 (d) O−C Bond of CH3OH −0.4767 −0.45256 −0.45120 −0.44862 −0.43828 −0.43632 −0.48286 −0.45626 −0.45439

λ3c

|λ1|/λ3

0.05803 0.02735 0.00336

0.165776 0.183181 0.089286

0.12211 0.12047 0.12099

0.919335 0.93268 0.918919

1.60456 1.60048 1.60289

1.123828 1.131554 1.126079

0.42706 0.42598 0.42779

1.056526 1.024273 1.06218

ρ(rc) is electron density. b∇2ρ(rc) is Laplacian of electron density. cλ1, λ2, and λ3 are three eigenvalues of Hessian matrix. dP−Cl···H−O H-bond interaction. eP−Cl···H−C H-bond interaction. a

Table 5. Electron Occupancies of Various NBOs of the Adducts A and B Computed at the B3LYP/aug-cc-pVDZ Level of Theorya adduct

NBO

adduct A

n(σ) O5 σ*(P7−Cl9) n(π) O5 σ*(P7−Cl9) n(σ) P7 σ*(C1−O5) σ(C1−H3) σ*(O5−H6) n(σ) Cl9 σ*(O5−H6) n(σ) O5 σ*(P7−Cl9) σ(C1−H2) σ*(O5−H6)

adduct B

occupancy 1.94400 0.11309 1.97957 0.11309 1.99488 0.00230 1.99124 0.00710 1.99659 0.01020 1.96109 0.10492 1.99157 0.01020

donor−acceptor delocalization interaction

(1.96157)b (0.10185)c (1.98402)b (0.10185)c (1.99668)c (0.00138)b (1.99152)b (0.00658)b (1.99728)c (0.00658)b (1.96157)b (0.10185)c (1.99152)b (0.00658)b

E2

n(σ) O5 → σ*(P7−Cl9)

3.60

n(π) O5 → σ*(P7−Cl9)

0.82

n(σ) P7 → σ*(C1−O5)

0.82

σ(C1−H3) → σ*(O5−H6)

2.61

n(σ) Cl9 → σ*(O5−H6)

1.11

n(σ) O5 → σ*(P7−Cl9)

0.05 kcal/mol. The possible reason for the variation in E2 energies could be due to the orientation of the chlorine atoms in phosphorus toward oxygen atoms in methanol. It was found in adduct A that the ∠O5−P7−Cl9 ≈ 175° is almost linear, which aids in effective overlap of orbital between donor and acceptor groups. However, ∠O5−P7−Cl8 and ∠O5−P7−Cl10 are almost perpendicular (80° and 85°, respectively), which supports the above observation; hence, the P7−Cl9 bond is different when compared to the other two P−Cl bonds. Furthermore, the electron transfer to the σ*(P7−Cl9) acceptor orbital leads to elongation of the P−Cl bond by 0.014 Ǻ with respect to the monomer. This elongation is responsible for the red-shift in the P−Cl stretching mode of adduct A. Surprisingly, the same magnitude of elongation is found in the case of P7− Cl8, whereas it is comparatively less in the case of P7−Cl10 by 0.008 Ǻ with respect to the monomer value. In adduct A, the second order perturbation E2 energy for the charge transfer n(σ) P7 → σ*(C1−O5) interaction was fond to be 0.82 Kcal/mol. Although, the magnitude of this interaction is small it cannot be neglected as this interaction leads to lengthening of the C-O bond and results in red shift. However, in the adduct B, such a charge transfer interaction is absent, and only a small decrease in the C-O bond length is observed, resulting in the blue shift of the C-O stretch, in comparison to the CH3OH monomer. In adduct B, the electron occupancy of the chlorine (Cl9) lone pair decreases by ∼0.0007e compared to the PCl3 monomer and the electron occupancy of σ*(O5−H6) increases

Table 6. Percentage d Character along the P−Cl Bond for the PCl3 Monomer, P-Bonded Adduct A, and H-Bonded Adduct B Using the B3LYP/aug-cc-pVDZ Level of Theory. bond

PCl3 monomer

bond

P-bonded adduct A

H-bonded adduct B

P1−Cl2 P1−Cl3 P1−Cl4

1.59 1.59 1.59

P7−Cl8 P7−Cl9 P7−Cl10

1.60 1.69 1.59

1.60 1.60 1.59

adduct A is enhanced by 0.1% compared to the PCl3 monomer. This clearly supports that the vacant ‘d’ orbital of phosphorus is participating in the P···O bonding. It is clear from the NBO analysis that the phosphorus atom acts as a bridge between oxygen of CH3OH and one of the chlorine atoms of PCl3, and hence, P-bonding should be considered as an H-bonding analogue. However, no enhancement in the %d character of phosphorus atom observed in the case of P7−Cl8 and P7− Cl10 bonds indicates the nonequivalence of the P−Cl bond and further the direction of interaction in adduct A. In adduct B the %d character on the phosphorus atom remained the same for all three P−Cl bonds. It should be mentioned that the charge-transfer interaction alone may not be sufficient for the description of weak interactions; other interactions like electrostatic attraction, polarization, and dispersion also can contribute to the stability of adducts. Scheiner showed computationally P···N interaction dominates over the H-bond interaction in the PH3-NH3 system wherein other interactions are also considered.53 In another study, Scheiner revealed that when one hydrogen in the PH3 is replaced by electron withdrawing substituent the strength of the P···N interaction increases.54 Energies for P···N interactions with different electron withdrawing substituents were determined, the strength of interaction in case of fluorine substituted adduct was reported to be the highest among all other substituents. This is mainly due to inductive and electrostatic energies with smaller contribution from dispersion. In the present study since chlorine is attached to phosphorus, the electron density is significantly pulled away from the phosphorus making it more electropositive which helps in stabilizing the phosphorus bonded adduct. 3.4.2.1. Comparison between POCl 3 :CH 3 OH and PCl3:CH3OH Adducts. The study on noncovalent interaction in POCl3:CH3OH adducts has recently been reported.92 The adduct stabilized by H-bonding interaction (between H of CH3OH and Cl of POCl3) was the global minimum, which was experimentally identified in the low temperature N2 matrix. Clearly, the pentavalent state of the phosphorus atom in the POCl3 molecule precludes the P-bonding interaction with CH3OH. 3447



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(3) Pimentel, G. C.; Sederholm, C. H. Correlation of Infrared Stretching Frequencies and Hydrogen Bond Distances in Crystals. J. Chem. Phys. 1956, 24, 639−641. (4) Huggins, C. M.; Pimentel, G. C. Systematics of the Infrared Spectral Properties of Hydrogen Bonding Systems: Frequency Shift, Half Width and Intensity. J. Phys. Chem. 1956, 60, 1615−1619. (5) Thiel, M. V.; Becker, E. D.; Pimentel, G. C. Infrared Studies of Hydrogen Bonding of Methanol by the Matrix Isolation Technique. J. Chem. Phys. 1957, 27, 95−99. (6) Thiel, M. V.; Becker, E. D.; Pimentel, G. C. Infrared Studies of Hydrogen Bonding of Water by the Matrix Isolation Technique. J. Chem. Phys. 1957, 27, 486−490. (7) Becker, E. D.; Liddel, U. Infrared spectroscopic studies of hydrogen bonding in methanol, ethanol, and tert-butanol. Spectrochim. Acta Part A 1957, 10, 70−84. (8) Noble, P. N.; Pimentel, G. C. Hydrogen Dichloride Radical: Infrared Detection through the Matrix Isolation Technique. J. Chem. Phys. 1968, 49, 3165−3169. (9) Bellamy, L. J.; Owen, A. J. A simple relationship between the infra-red stretching frequencies and the hydrogen bond distances in crystals. Spectrochim. Acta Part A 1969, 25, 329−333. (10) Bellamy, L. J.; Pace, R. J. The significance of infra-red frequency shifts in relation to hydrogen bond strengths. Spectrochim. Acta Part A 1969, 25, 319−328. (11) King, S. T. Low-temperature matrix isolation study of hydrogenbonded, high-boiling organic compounds. I. Sampling device and the infrared spectra of pyrazole, imidazole, and dimethylphosphinic acid. J. Phys. Chem. 1970, 74, 2133−2138. (12) Ault, B. S.; Pimentel, G. C. Infrared Spectrum of WaterHydrochloric Acid Complex in Solid Nitrogen. J. Phys. Chem. 1973, 77, 57−61. (13) Ault, B. S.; Steinback, E. Matrix Isolation Studies of Hydrogen Bonding. Vibrational Correlation Diagram. J. Phys. Chem. 1975, 79, 615−620. (14) Engdahl, A.; Nelander, B. The acetylene-water complex: A matrix isolation study. Chem. Phys. Lett. 1983, 100, 129−132. (15) Rozenberg, M.; Shoham, G.; Reva, I.; Fausto, R. A correlation between the proton stretching vibration red shift and hydrogen bond length in polycrystalline amino acids and peptides. Phys. Chem. Chem. Phys. 2005, 7, 2376−2383. (16) Sundararajan, K.; Sankaran, K.; Viswanathan, K. S.; Kulkarni, A. D.; Gadre, S. R. H−π Complexes of Acetylene−Ethylene: A Matrix Isolation and Computational Study. J. Phys. Chem. A 2002, 106, 1504− 1510. (17) Sundararajan, K.; Viswanathan, K. S.; Kulkarni, A. D.; Gadre, S. R. H···π complexes of acetylene-benzene: a matrix isolation and computational study. J. Mol. Struct. 2002, 613, 209−222. (18) Sundararajan, K.; Ramanathan, N.; Viswanathan, K. S.; Vidya, K.; Jemmis, E. D. Complexes of acetylene-fluoroform: A matrix isolation and computational study. J. Mol. Struct. 2013, 1049, 69−77. (19) Jemmis, E. D.; Subramanian, G.; Nowek, A.; Gora, R. W.; Sullivan, R. H.; Leszczynski, J. C-H···π interactions involving acetylene: an ab initio MO study. J. Mol. Struct. 2000, 556, 315−320. (20) Ault, B. S.; Pimentel, G. C. Infrared Spectra of the Ammoniahydrochloric acid Complex in Solid Nitrogen. J. Phys. Chem. 1973, 77, 1649−1653. (21) Mielke, Z. Infrared Matrix-Isolation Studies of Complexes between Pyridine N-oxide and Hydrogen Chloride. J. Phys. Chem. 1984, 88, 3288−3292. (22) Del Bene, J. E.; Person, W. E.; Szczepaniak, K. Ab-initio theoretical and matrix-isolation experimental studies of hydrogenbonding: vibrational consequences of proton position in 1:1 complexes of HCl and 4-x-pyridines. Chem. Phys. Lett. 1995, 247, 89−94. (23) Del Bene, J. E.; Person, W. E.; Szczepaniak, K. Ab-initio theoretical and matrix-isolation experimental studies of hydrogenbonding, theoretical-study of distances, force-constants, and vibrational frequencies in complexes of hydrogen halides and 4-substituted pyridines. Mol. Phys. 1996, 89, 47−59.

Importantly, in the present study of PCl3:CH3OH, the adduct stabilized through H-bonding is the local minimum, and the P-bonded adduct was the global minimum, which indicates that the latter is stronger than the former. The trivalent state of the phosphorus atom directs the adduct to be stabilized by Pbonding.

4. CONCLUSIONS In the present study, we have provided experimental evidence for the noncovalent P···O interaction in PCl3−CH3OH adducts using matrix isolation infrared spectroscopy. Computations predicted the P-bonded adduct to be more stable than the Hbonded adduct. The formation of the P-bonded adduct was evidenced from the red shift in the P−Cl stretching mode of the PCl3 submolecule and red shift in O−C and O−H stretching modes of the CH3OH submolecule. Experimental frequencies correlated well with computational results. AIM analysis showed both the adducts are stabilized by weak interactions. The higher value of electron density (ρ(rc)) and Laplacian of electron density (∇2ρ(rc)) at the BCPs in the Pbonded adduct compared to the H-bonded adduct showed the P···O interaction is stronger than the conventional H-bonding interaction. NBO analysis revealed that hyperconjugative interaction is stronger in the P-bonded adduct than in the Hbonded adduct.

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ASSOCIATED CONTENT

S Supporting Information *

S(I) Cartesian coordinates of B3LYP/aug-cc-pVDZ, B3LYP/6311++G(d, p), and MP2/6-311++G(d, p) levels of theory for PCl3 monomer, PCl3 dimer, CH3OH monomer, CH3OH dimer, adduct A, and adduct B. S(II) computational vibrational frequencies for different isotopic possibilities of the PCl3 monomer, PCl3 dimer, CH3OH monomer, CH3OH dimer, adduct A, and adduct B calculated at B3LYP/6-311++G(d, p) and MP2/6-311++G(d, p) levels of theory. S(III) Figures: (i) Stacked mode spectra in the range 520−470 cm−1 in nitrogen matrix, (ii) stacked mode spectra in the range 3670−3640 and 1060−1020 cm−1 in the nitrogen matrix, (iii) P−Cl stretching mode in the Ar matrix, and (iv) complete range IR spectra in the nitrogen matrix. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 044 27480098. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.R.J. is thankful to IGCAR, Department of Atomic Research, India for providing Research Associate fellowship. The authors thank the reviewers for critically and meticulously analyzing the work which resulted in constructive improvement in the manuscript.

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REFERENCES

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