Positively Charged Phosphorus as a Hydrogen Bond Acceptor - The

Letter pubs.acs.org/JPCL

Positively Charged Phosphorus as a Hydrogen Bond Acceptor Anne S. Hansen, Lin Du, and Henrik G. Kjaergaard* Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark S Supporting Information *

ABSTRACT: Phosphorus (P) is an element that is essential to the life of all organisms, and the atmospheric detection of phosphine suggests the existence of a volatile biogeochemical P cycle. Here, we investigate the ability of P to participate in the formation of OH···P hydrogen bonds. Three bimolecular alcohol−trimethylphosphine complexes have been detected. Initially, the complexes were detected using matrix isolation spectroscopy, which favors complex formation. Subsequently, the fundamental OH-stretching vibration was observed in room-temperature gas-phase spectra. On the basis of our measured OH-stretching frequency red shifts and quantum chemical calculations, we find that P is an acceptor atom similar in strength to O and S and that all three P, O, and S atoms are weaker acceptors than N. The quantum chemical calculations show that both H and P in the OH···P hydrogen bond have partial positive charges, as expected from their electronegativities. However, the electrostatic potentials show a negative potential area on the electron density surface around P that facilitates formation of hydrogen bonds. SECTION: Spectroscopy, Photochemistry, and Excited States

H

alcohols used as hydrogen bond donors were 2,2,2-trifluoroethanol (TFE), ethanol (EtOH), and methanol (MeOH). We compare the experimental observation of the OH···P hydrogen bond to the OH···Y (Y = N, O, S) hydrogen bonds, which have been observed previously.21−25 Quantum chemical calculations were performed in order to obtain binding energies, atomic charges,26 and electrostatic potential surfaces (EPSs)27 of the OH···P hydrogen-bound complexes. DFT local mode OHstretching intensities were also calculated28−30 and combined with the measured gas-phase OH-stretching intensity to determine the equilibrium constants of complexation.21,31 The OH···P hydrogen bond in the alcohol−TMP complexes (Figure 1 and Tables S1 and S2, Supporting Information (SI)) was initially detected with matrix isolation Fourier transform infrared spectroscopy (FTIR) spectroscopy. The jet expansion into the vacuum chamber favors the formation of complexes. In Figure 2, we show the Ar matrix spectra of the alcohols, TMP, and alcohol−TMP mixtures. In the spectra of the mixtures, new bands appear that are not present in the monomer spectra, indicating complex formation. Annealing experiments were performed and show growth of larger clusters as expected (Figure S1, SI). Alcohols are known to form dimers and larger clusters in jets.32−41 In the matrix spectra of the pure alcohols, red-shifted OH-stretching transitions from the alcohol complexes are observed at wavenumbers in agreement with literature values (Figure S2, SI).42−51 Due to the low pressure of alcohol used in the matrix spectra, transitions from the

ydrogen bonds (XH···Y) are partly responsible for the formation of atmospheric aerosol particles,1,2 which affect climate change3,4 and impact human health.5 Even though the atmospheric aerosol nucleation has received a lot of attention, uncertainties about the nucleation mechanisms and the species involved still exist. The presence of phosphine in the upper and lower atmosphere suggests that a volatile P biogeochemical cycle exists.6−11 P is also an element that is essential to the life of all organisms and very important in biomolecules with adenosine triphosphate, a well-known example.12 Here, we focus on the possibility of formation of a OH···P hydrogen bond and its characterization. In the recent IUPAC recommendation,19,20 it was stated that X should be more electronegative than H, but no such restriction was placed on Y. In the OH···P hydrogen bond in the studied alcohol− trimethylphosphine (TMP) complexes, P is less electronegative than C, and a partial positive charge is expected on both the H and P atoms. Noncovalent interactions, such as hydrogen bonds, play a key role in biology,13,14 and theoretical studies of various OH···P hydrogen bond systems exist.6,15,16 In addition, detection of several OH···Y hydrogen bonds in bimolecular complexes has been studied, where Y = N, O, or S. However, to our knowledge, the OH···P hydrogen bond in a bimolecular complex has not yet been observed. Previously, experimental studies of ethanol (and water) dissolved in liquid hydrides, including phosphine, have suggested OH to P hydrogen bond interaction.17,18 Here, we present Ar matrix and room-temperature gas phase spectra of the bimolecular complexes formed between a series of alcohols and TMP, which provide unequivocal experimental evidence for the formation of a OH···P hydrogen bond. The © 2014 American Chemical Society

Received: October 9, 2014 Accepted: November 19, 2014 Published: November 19, 2014 4225

dx.doi.org/10.1021/jz502150d | J. Phys. Chem. Lett. 2014, 5, 4225−4231

The Journal of Physical Chemistry Letters

Letter

Figure 2. Matrix FTIR spectra of alcohol, TMP, and mixtures recorded at 12 K. The mixing ratios were alcohol/TMP/Ar in black 1:5:720 Torr; alcohol/Ar in green 1:720 Torr; TMP/Ar in orange 5:720 Torr.

Table 1. Observed OH-Stretching Frequencies and Red Shifts (cm−1) in Gas-Phase and Ar Matrix Experiments for the Alcohol−TMP Complexes, the Alcohol Dimers, and the Alcohol Monomers along with the Determined Equilibrium Constants for the Alcohol−TMP Complexes

Figure 1. Optimized M06-2X/aug-cc-pV(T+d)Z structures for the alcohol−TMP complexes. The wB97XD/aug-cc-pV(T+d)Z and B3LYP/aug-cc-pV(T+d)Z structures are similar to those shown here.

gas

alcohol dimers are insignificant in the spectra of the alcohol− TMP mixtures shown in Figure 2. In Table 1, the OHstretching frequencies observed in the matrix experiments are summarized. In gas-phase spectroscopy, only binary complexes are formed, and matrix effects are avoided.52 In Figure 3, we show the gas-phase spectra of the three alcohol−TMP complexes in the OH-stretching region. Two different combinations of monomer pressures were used to give the two spectra (black and red) displayed. Because small amounts of alcohol dimers are also formed, we recorded spectra of these dimers to ensure that they do not interfere with our observed OH-stretching vibration of the alcohol−TMP complexes. The MeOH dimer, (MeOH)2, spectrum (Figure 4) is in agreement with previous gas-phase (MeOH) observations.53 The alcohol dimer spectra were obtained by recording an alcohol spectrum at approximately 1 Torr, where almost no dimer is present, and subtracting this spectrum from alcohol spectra recorded at higher pressures, 30−70 Torr.54 The alcohol dimers have previously been observed using jet-cooled spectroscopy,32−41 and the OH-stretching transitions were observed at a slightly lower frequency than the band maxima in the present roomtemperature spectra (Figure S3, SI).55 The OH-stretching frequencies in the alcohol monomers are also shown in Figure S3 (SI). In Figure 4, we compare the OH-stretching vibration

a

Ar matrix

ν̃OH

Δν̃OHa

Keq

ν̃OH

Δν̃OHa

TFE·TMP EtOH·TMP MeOH·TMP

3390 3531 3541

267 146 140

0.13 0.010 0.012

336 208, 216 190−223

(TFE)2

3566

91

(EtOH)2

3578

99

(MeOH)2

3599

82

TFE EtOH MeOH

3657 3677 3681

3305 3450, 3442 3476, 3467, 3462, 3443 3545, 3537, 3532, 3520, 3506, 3499, 3494 3536, 3531, 3527, 3523 3541, 3533, 3527, 3518 3639, 3626 3660, 3656 3666

94−145

125−137 125−148

Δν̃OH = ν̃monomer − ν̃complex.

in the newly found MeOH·TMP complex to that in (MeOH)2 and the MeOH monomer. The OH-stretching vibration in (MeOH)2 and MeOH·TMP are close but clearly distinct. Table 1 summarizes the observed gas-phase OH-stretching frequencies. The spectra of the OH-stretching vibration in the EtOH· TMP and MeOH·TMP complexes are very similar, which indicate that replacing a H with a CH3 group has minimal effect 4226



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S9 (SI).26,27 On the basis of the electronegativity of O, H, P, and C, we expect partial positive charges on H and P in agreement with the calculated ATP charges shown in Figure 5.

Figure 5. EPSs for MeOH, TMP, and the MeOH·TMP complex calculated with the M06-2X/aug-cc-pV(T+d)Z method. Blue and red regions represent positive and negative potential areas, respectively. Calculated ATP charges are given.

After complex formation, the APT charges on H and P become slightly more positive. The formation of a hydrogen bond between partial positive charged atoms is not the common definition for the origin of hydrogen bonds. However, hydrogen bonding occurring between ions alike have very recently been investigated theoretically, and it was found that hydrogen bonds can occur between both anion−anion and cation−cation types of complexes.56 EPSs of the MeOH donor and the TMP acceptor are shown in Figure 5, with blue and red regions representing positive and negative potential areas, respectively. The EPSs clearly show that there is a negative potential area at P, and therefore, TMP can act as a hydrogen bond acceptor. In the IUPAC definition of the hydrogen bond, it is stated that X must be more electronegative than H, whereas for Y, it is stated that it must have some region of negative electron density.19,20 This is in agreement with our observations for the alcohol−TMP complexes. The measured alcohol−TMP OH-stretching intensities (integrated absorbance, ∫ A(υ̃) dυ̃) were used to determine the equilibrium constants (Keq) of the complexes using a method described previously.21,31 The pressure of the alcohol− TMP complexes in the experiment is determined from

Figure 3. Observed OH-stretching vibration in the alcohol−TMP complexes recorded in the gas phase. The two spectra (black and red) differ in the pressure of the alcohol and TMP monomers and, hence, the amount of complex. See SI Figure S3 for details.

Pcomplex = 2.6935 × 10−9 (K−1 Torr m cm)

T ∫ A(υ)̃ dυ ̃ fcalc × l (1)

where T is the temperature, l the optical path length, and fcalc the local mode calculated intensities, which are given in Tables S3 and S4 (SI). The equilibrium constant is found from

Figure 4. Spectra of the OH-stretching vibration in the MeOH monomer (green), MeOH dimer (blue), and the MeOH·TMP complex (red), which were recorded with 4.8 m, 10 cm, and 6 m cells, respectively. The MeOH dimer spectrum is noisy due to incomplete subtraction. The MeOH−TMP and (MeOH)2 spectra have been offset for clarity.

Keq =

Pcomplex /P ⊖ ⊖

PA /P × PB/P

⊖

=

Pcomplex PAPB

× P⊖ (2)

where Pcomplex is determined from eq 1, PA and PB are the pressure of the monomers, and P⊖ is the standard pressure, 1 bar. The calculated OH-stretching intensities for the EtOH· TMP complex are relatively independent of the two conformers, and an average intensity is used in eq 1 (see Tables S4 and S5, SI). In Figure 6, the complex pressure is plotted as a function of the product of the two monomer pressures. The slope of the fitted straight line gives Keq for the three alcohol− TMP complexes (Table 1). For the EtOH·TMP and MeOH· TMP complexes, similar Keq’s are obtained, whereas for the

on the hydrogen bond formed. However, replacing a H with a CF3 group changes the OH-stretching transition significantly. The wavenumber shifts, and the OH-stretching bandwidth increases, which agrees with our previous observations in stronger bound alcohol−amine complexes.31 To understand the formation of a hydrogen bond, OH···P, in the observed alcohol−TMP complexes, we have calculated the atomic polar tensor (APT) charges and EPSs; see Figures S4− 4227



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were acting as hydrogen bond donors, forming complexes with TMP, TMA, DME, and DMS. With 3,3,3-trifluoroprop-1-yne as the AL hydrogen bond donor, the general order of the hydrogen bond strength based on the CH-stretching red shifts was found to be AL·TMA (288 cm−1) > AL·TMP (146 cm−1) ≳ AL·DME (125 cm−1) ≳ AL·DMS (127 cm−1).59 The observed CH-stretching red shifts for the complexes with TMP, DME, and DMS as hydrogen bond acceptors were similar, and the three complexes form hydrogen bonds of similar strength. In the more recent study, HAL was the hydrogen bond donor forming complexes with TMP-d9, TMA-d9, and DMS-d6, which were compared to previous measurements of the HAL·DME-d6 complex.57,60 From the CH-stretching red shifts, the hydrogen bond strength order of the complexes was found to be HAL· TMA (126 cm−1) > HAL·TMP (64 cm−1) ≳ HAL·DMS (43 cm−1) ≳ HAL·DME (19 cm−1). Again, the observed CHstretching red shifts for the complexes with TMP, DMS, and DME as hydrogen bond acceptors were similar. We have detected the weak OH···P hydrogen bond in three alcohol−TMP complexes using gas-phase FTIR spectroscopy. As expected from the electronegativities of the atoms involved, we found that both the H and P atoms in the OH···P hydrogen bond have positive charges. However, from the EPSs, it is seen that a negative potential area is found around the lone pair of P, which facilitates formation of hydrogen bonds. Equilibrium constants were determined from the observed and calculated OH-stretching intensities for the alcohol−TMP complexes. The observed OH-stretching red shift and equilibrium constants were compared to previous observations for OH··· N, OH···O, and OH···S hydrogen-bound complexes. We found that the OH···P hydrogen bond is similar in strength to OH···O and OH···S, but all three are weaker than the OH···N hydrogen bond.

Figure 6. Pressure of the complex as a function of the multiplied pressure of the monomers. Oscillator strengths of 2.37 × 10−4, 1.42 × 10−4, and 1.47 × 10−4 for the fundamental OH-stretching transitions were used to determine Pcomplex for the TFE·TMP, EtOH·TMP, and MeOH·TMP complexes, respectively. Error bars representing the experimental uncertainty (±20%) are included.

TFE·TMP complex, a much larger Keq value is obtained. These Keq values for the alcohol−TMP complexes are about an order of magnitude smaller than those found previously for alcoholtrimethylamine (TMA) complexes.31 In Table 2, the M06-2X/ aug-cc-pV(T+d)Z binding energies (BEs) for the complexes MeOH·TMP, MeOH·TMA, MeOH-dimethyl ether (DME), and MeOH-dimethylsulfide (DMS) are given. The M06-2X/ aug-cc-pVTZ method has previously been shown to calculate binding energies of hydrogen-bound complexes, in good agreement with those obtained with higher-level CCSD(T)F12a/VDZ-F12 calculations.6,21 The complexes MeOH·TMP, MeOH·DME, and MeOH·DMS have similar BEs, thus forming hydrogen bonds of similar of strength, which agree with the similarity of the observed OH-stretching red shifts. In Table 2, we also compare the observed gas-phase OHstretching red shift for the MeOH·TMP complex to red shifts previously observed for the complexes MeOH·TMA, MeOH· DME, and MeOH·DMS.21,22 On the basis of the red shifts, the order of the hydrogen bond strength is MeOH·TMA (333 cm−1) > MeOH·TMP (140 cm−1) ≳ MeOH·DMS (113 cm−1) ≳ MeOH·DME (103 cm−1), which shows a similar trend to that in the previous observation for the halothane (HAL) complexes.57 Previously, the OH···O and OH···S hydrogen bond strengths were investigated for the p-cresol−water and pcresol−hydrogen sulfide complexes.58 The OH-stretching red shifts for the complexes were within 20 cm−1 of each other, which supports that OH···O and OH···S are hydrogen bonds of similar strength. The CH···P type of hydrogen bond has previously been observed.57,59,60 In an Ar matrix study, different alkynes (AL)

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EXPERIMENTAL AND COMPUTATIONAL DETAILS MeOH (Aldrich anhydrous, 99.8%), EtOH (Kemetyl anhydrous, 99.9%), TFE (Aldrich anhydrous, 99.9%), and TMP (Aldrich anhydrous, 97%) were purified by freeze, pump, and thaw cycles. The room-temperature (298 K) gas-phase IR spectra were recorded with a VERTEX 70 (Bruker) FTIR spectrometer using a 1 cm−1 resolution and 500 scans. A CaF2 beam splitter, liquid N2-cooled MCT detector, and MIR light source were fitted to the spectrometer, which was purged with dry nitrogen gas to minimize the interference by H2O and CO2 vapors. A 10 cm path length gas cell equipped with KBr windows and 4.8 and 6 m multireflection White cells (Infrared Analysis, Inc.) equipped with KCl windows and gold-coated mirrors were used to measure the spectra. The alcohol−TMP mixtures were prepared in the 6 m multireflection gas cell, which was connected to a glass vacuum line with a base pressure of 1 × 10−4 Torr. Sample pressures were measured with a Varian diaphragm pressure gauge (5 × 10−5−1500 mbar, PCG-750) connected to the vacuum line.

Table 2. Observed Gas-Phase OH-Stretching Frequencies and Red Shifts (cm−1) and Calculated M06-2X/aug-cc-pV(T+d)Z Binding Energies Including Zero-Point Vibrational Energy (kcal/mol) for Different MeOH Complexes MeOH·TMAa

MeOH·TMP

a

MeOH·DME

MeOH·DMS

ν̃

Δν̃

BE

ν̃

Δν̃

BE

ν̃b

Δν̃b

BE

ν̃b

Δν̃b

BE

3541

140

4.24

3355

333

6.74

3578

103

4.75

3568

113

4.78

Taken from refs 21 and 31. bTaken from ref 22. 4228



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The matrix isolation spectra were recorded with our Ar matrix isolation FTIR setup. Low temperatures were achieved using a closed-cycle helium-compressor-cooled cryostat (CS202SI, Advanced Research Systems, Inc.), which was housed in a vacuum chamber with a base pressure of less than 5 × 10−6 Torr, measured with a compact full-range gauge (PKR 250, Balzers). High-purity Ar (Air Liquide, ≥99.999%) was used as the matrix gas. Alcohol−TMP−Ar mixtures were prepared at room temperature, with different mixing ratios (1:5:720 and 5:5:720 Torr), in a 1 L glass bulb connected to a vacuum line (base pressure of 1 × 10−4 Torr). The gas mixture was then deposited onto a CaF2 substrate maintained at 12 K. The temperature of the substrate was measured with a silicon diode sensor (LakeShore) and regulated by a temperature controller (model 32, Cryocon). The flow of the gas deposition was controlled by a leak valve (model 203021, Brooks). The deposition was performed at a rate of approximately 20 mmol/ h and lasted for about 60 min. Infrared spectra of the matrixisolated samples were recorded using a VERTEX 80 (Bruker) FTIR spectrometer fitted with the same beamsplitter, detector, and light source as the VERTEX 70. Spectra were recorded with a 0.5 cm−1 resolution and 256 scans. After recording a spectrum at 12 K, the matrix was annealed to 25 K, kept at that temperature for 20 min, and then cooled back to 12 K, and a second spectrum was recorded. Finally, the matrix was annealed to 35 K and cooled back to 12 K, and a third spectrum was recorded. All spectral subtraction and analyses were performed with OPUS 6.5 and Origin 8.1 software (Figure S10, SI). Quantum chemical calculations were performed to determine the number of hydrogen-bound conformers and the corresponding binding energies. The geometries of the monomers and complexes were optimized in Gaussian 0961 with the B3LYP,62 wB97XD,63 and M06-2X64 functionals and the augcc-pV(T+d)Z basis set, using the options “opt=verytight” and “integral=ultrafine”. The optimization was followed by a harmonic frequency calculation to ensure that an energy minimum was found. From the optimizations, APT charges26 and EPSs27 were also obtained. The OH-stretching oscillator strengths in the alcohol−TMP complexes were calculated with an anharmonic oscillator local mode model.28−30 The potential energy grid was calculated with the B3LYP/aug-cc-pV(T+d)Z method because it has previously been found to calculate intensities in good agreement with experimental data.31 Details on the anharmonic oscillator local mode model have been described in detail elsewhere.31

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +45-35320334. Fax: +4535320322. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor E. Arunan, Professor Kurt V. Mikkelsen, Kasper Mackeprang, Sidsel D. Schrøder, and Sofie N. Gottfredsen for helpful discussions. We acknowledge the financial support from The Danish Council for Independent Research - Natural Sciences and the Danish Center for Scientific Computing.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized geometric parameters; calculated binding energies; calculated Gibbs free energies; calculated equilibrium constants; calculated OH-stretching anharmonic parameters; calculated anharmonic OH-stretching frequencies; calculated abundance for EtOH·TMP gauche and trans conformers; matrix spectra of alcohol−TMP−Ar mixtures; matrix spectra of alcohol−Ar mixtures; gas-phase spectra of the OH-stretching transition in alcohol−TMP complexes, alcohol dimers, and alcohol monomers; atomic polar tensor charges; electrostatic potential surfaces; peak fitting; and comparison of MeOH gas-phase and matrix spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 4229



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