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A Spectroscopic and Computational Investigation of the Conformational Structural Changes Induced by Hydrogen Bonding Networks in the Glycidol-Water Complex A. R. Conrad, N. H. Teumelsan, P. E. Wang, and M. J. Tubergen* Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed: August 29, 2009; ReVised Manuscript ReceiVed: October 21, 2009
Rotational spectra were recorded in natural abundance for the 13C isotopomers of two conformers of glycidol. Moments of inertia from the 13C isotopomers were used to calculate the substitution coordinates and C-C bond lengths of two glycidol monomer conformations. The structures of seven different conformational minima were found from ab initio (MP2/6-311++G(d,p)) optimizations of glycidol-water. The rotational spectrum of glycidol-water was recorded using microwave spectroscopy, and the rotational constants were determined to be A ) 3902.331 (11) MHz, B ) 2763.176 (3) MHz, and C ) 1966.863 (3) MHz. Rotational spectra were also recorded for glycidol-H218O, glycidol-DbOH, and glycidol-dO-D2O. The rotational spectra were assigned to the lowest-energy ab initio structure, and the structure was improved by fitting to the experimental moments of inertia. The best-fit structure shows evidence for structural changes in glycidol to accommodate formation of the intermolecular hydrogen bonding network: the O-C-C-O torsional angle in glycidol was found to increase from 40.8° for the monomer to 49.9° in the water complex. Introduction Hydrogen-bonded complexes involving water can be formed in adiabatic expansions and probed spectroscopically; the hydrogen bonding interactions in these complexes are thought to model the more complicated hydrogen bonding networks observed in biological systems.1 Rotationally resolved spectroscopy, in particular, has been used to obtain precise measures of hydrogen bond lengths and to investigate tunneling dynamics within hydrogen-bonded complexes.2-7 Past work in our laboratory has focused on the rotational spectra and structures of hydrogen-bonded complexes of biological interest including alaninamide-water8 and 3-hydroxytetrahydrofuran-water.9 Spectroscopically determined structures of these weakly bonded complexes usually employ the assumption that the structures of the individual monomer units are unchanged upon formation of the complex, even if new hydrogen bonding interactions are formed. In many biological systems, however, the lowest-energy structure is not suitable for forming the strongest intermolecular hydrogen bonds, and the structure of the biomolecule depends on the environment. The large-scale structural changes observed for proteins upon agonist binding provide dramatic examples of intermolecular interactions altering the relative stabilities of molecular conformations, but conformation changes may also be observed in smaller molecules. The “alanine dipeptide” (Nacetyl-alanine N′-methylamide) model peptide system adopts a seven-membered ring structure (C7eq) in the gas phase,10 but the polyproline-II conformation (PII) is more stable in both the crystal11,12 and aqueous solution phases.13 Ab initio calculations indicate that the polyproline-II conformation may be stabilized by as few as four water molecules forming a hydrogen-bonding bridge.14 We have also used high-resolution rotational spectroscopy to characterize a change in the structure of 2-aminoethanol upon complexation with water.15 Moments of inertia, derived from fitting the rotational transitions of the complex, were assigned to a structure in which the aminoethanol OCCN * Corresponding author. E-mail:
[email protected].
dihedral angle increased from 57° (monomer)16 to 75° (complex). A new network of intermolecular hydrogen bonds is formed in the water complex, and the structure of the 2-aminoethanol adjusts to accommodate formation of the new hydrogen bonds; formation of the bridging hydrogen bonds liberates sufficient energy to drive the structural changes in the monomer. UV and UV-IR double resonance techniques have revealed similar structural changes in both 2-amino-1-phenylethanol17 and pseudoephedrine18 upon formation of their 1:1 water complexes. Glycidol, also known as 2-oxiranemethanol, is a small molecular ring system that is important as an enantioselective synthetic reagent;19 it has also been used as a model system for gas-phase spectroscopic studies of chiral self-recognition.20 Rotational spectra arising from the two lowest-energy glycidol conformations were assigned in the microwave spectrum of the monomer.21,22 These structures can be distinguished by the orientation of the hydroxyl group: above the oxirane ring (“inner”; conformer I), or outside the ring (“outer”; conformer II) adjacent to the oxirane O-C bond. Hartree-Fock-level calculations with the 6-31G(d) basis set identified conformer I, with its intramolecular hydrogen bond from the hydroxyl group to the oxirane ring oxygen, as the most stable structure of the glycidol monomer.22 In conformer II, the intramolecular hydrogen bond is from the hydroxyl group to the pseudo-π electrons of the O-C bond. This hydrogen bond is weaker than that found in I, and HF/6-31G(d) calculations predict conformer II to be 4.5 kJ mol-1 less stable than I.22 A third conformational structure of glycidol (III) was identified by the ab initio calculations, but it could not be identified in the microwave spectrum. Conformer III is stabilized by an even weaker intramolecular hydrogen bond from the hydroxyl to the pseudo-π electrons of the oxirane C-C bond; this structure was calculated to be 10.3 kJ mol-1 less stable than I. The structures of the three glycidol monomer conformations are shown in Figure 1. A better understanding of the changes in the structure of glycidol upon forming new hydrogen bonds in glycidol-water
10.1021/jp908351u 2010 American Chemical Society Published on Web 11/11/2009
H-Bonding Networks in the Glycidol-Water Complex
Figure 1. Ab initio structures of glycidol conformations.
will be helpful to fully understand the structure of glycidol within diastereomeric glycidol dimers. Experimental Method Rotational spectra of glycidol and glycidol-water were recorded using a new “minicavity” Fourier-transform microwave spectrometer23 designed after those at NIST.24 The vacuum chamber housing the spectrometer consists of a six-way cross formed by a 15.5 in. long, 8 in. diameter tube with four 6 in. diameter ports. A 15 in. long arm extends the 8 in. diameter tube; this extension houses the motion platform for the movable cavity mirror (see below). The chamber is pumped by a Varian VHS-6 diffusion pump (2400 L s-1) backed by a two-stage Edwards E2M30 rotary pump. A 6 in. gate valve and diffusion pump bypass system allow the chamber to be isolated from the diffusion pump for rapid venting and chamber access. Easy chamber access is desirable for applications involving our new laser vaporization source.25 A Fabry-Perot resonant cavity is established by two 7.5 in. diameter spherical (spherical radius of curvature ) 30.5 cm) aluminum mirrors; the mirrors were finish polished by diamondtip machining. One mirror is formed on an 8 in. diameter flange and mounted on the six-way cross, while the second mirror is movable because it is mounted on 0.75 in. diameter steel rails that pass through ball bearing brackets mounted in the extension arm. A motorized micrometer is used to position the movable mirror over a two-inch range of travel; the two mirrors are nominally separated by 30 cm. A recessed region on the back of the stationary mirror accommodates either a standard Series 9 General Valve or a reservoir nozzle26,27 using the solenoid and armature of a modified General Valve. These valves can be heated to approximately 200 °C, if necessary, by Watlow band heaters (STB1A1A3-A12) and an Omega CN8201 temperature controller. The expansion passes through a 0.182 in. diameter hole in the mirror into the resonant cavity. The center of the expansion is offset from the center of the mirror by 1 in. The microwave circuit of our new spectrometer is analogous to that described for the NIST minicavity FTMW instrument;24 a schematic of our circuit is shown in Figure 2. An Agilent Technologies E8247C PSG CW synthesizer serves as our frequency source. Microwave radiation, typically 15 dBm, from the synthesizer is routed into a Sierra Microwave Technologies SPDT pin diode switch (S1; SFD0526-011). During the irradiation pulse, this switch is closed to the irradiation half of the
J. Phys. Chem. A, Vol. 114, No. 1, 2010 337 microwave circuit (S1a), and the microwave output is routed into a Miteq single sideband mixer (SSBM: SM0226LC1A). The IF for the SSBM is provided by the 10 MHz output of the Agilent generator, which has been frequency multiplied to 30 MHz by a Techtrol Cyclonetics frequency multiplier (FXA21730). A Minicircuits SPST pin diode switch (S2; ZYSW-2-50DR) passes the 30 MHz sideband into the SSBM mixer only during the irradiation pulse. The output of the SSBM mixer, offset by +30 MHz from the frequency generator, is amplified by 18 dB by a low-noise amplifier (A1; Miteq AFSM3-02001800-40-8PC) and admitted into the resonant cavity through an SPDT diode switch (S3; Sierra SFD0526-011); the SPDT switch is located outside the chamber, immediately before the L-shaped antenna which couples the microwave radiation into the cavity. The irradiation pulses typically have a duration of 0.8 µs. The detection arm of the microwave circuit is typically closed for 3-5 µs after the irradiation pulse ends, allowing cavity ringing to decrease. The second branch of the SPDT switch (S3b) is closed allowing any free induction signal to be amplified by a low noise amplifier (A2; Miteq JS4-10002600-22-5A; 33 dB gain, 2.2 dB noise figure; located immediately after the SPDT switch) and passed into an image rejection mixer (IR: Miteq IR0226LC1A). Switch S1b also closes during detection to allow the original generator frequency into the local oscillator port of the image rejection mixer. The IF output of the mixer is the free induction decay signal superimposed on the 30 MHz sideband frequency; this output is filtered (Minicircuits SIF-30 band-pass filter), amplified by a Miteq Au-1494 amplifier (A3; 56 dB gain), and routed into a National Instruments 100 MHz, 8-bit digitizing board (NI 5112). 40 000 channels are digitized at the 100 MHz rate, resulting in a digital frequency resolution of 2.5 kHz. Timing of the gas pulse, irradiation, and detection pulses is controlled by two Stanford Research Systems Digital Delay Generators (DG535); these delay generators are phase locked to the Agilent frequency generator through the 10 MHz reference signal. Decreasing cavity Q establishes the low end of the spectrometer’s range at 10.5 GHz. The low-noise amplifier A1 falls off above 20 GHz, although some transitions of difluorocyclopropane have been recorded up to 23 GHz.28 The upper limit of the spectrometer’s range could be extended to 26 GHz by replacing A1 with an amplifier rated for higher frequencies. Customized Labview software was written to control the frequency and digital delay generators (through a National Instruments GPIB board; 778032-01), perform signal averaging, display and save the transformed spectral data, and scan the spectrometer by stepping the frequency source and cavity. The software adjusts the cavity position by sweeping the frequency while recording the voltage output of a Herotek Inc. DHM265AA Schottky detector using the second channel of the digitizing board (Figure 2). A National Instruments PCI-6601 countertimer board powers the motorized micrometer until the frequency of the cavity is resonant with the irradiation frequency. The software also includes functions to control timing of a laser pulse for laser vaporization experiments.25 The instrument has the sensitivity to detect the 5/2 r 5/2 and 7/2 r 5/2 components of the 1 r 0 transition of 17O13CS in natural abundance in a sample made 2% OCS in Ar within 1000 shots. Figure 3 shows a representative signal assigned to the 414-313 transition of glycidol-water; this signal is the result of averaging 400 shots at 16895.100 MHz. Because the supersonic expansion proceeds through one of the cavity mirrors and along the cavity axis, rotational transitions are split into Doppler doublets centered at the transition frequency. The fwhm
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Figure 2. Schematic of the microwave circuit for the Fourier-transform microwave spectrometer. See text for a description of the components.
Results and Structures
Figure 3. Portion of the microwave spectrum of glycidol-water showing the 414-313 transition. This spectrum is the average of 400 gas pulses. Two Doppler components of the single rotational transition (16895.415 MHz) arise from the parallel orientation of the molecular beam with respect to the cavity axis.
of each Doppler component is typically 13 kHz. The vacuum system can accommodate pulse repetition rates of up to 15 s-1 while maintaining a pressure below 10-4 torr, and the instrument can scan 450 MHz in 6 h while averaging 100 shots per scan segment. Glycidol was used as received from Aldrich. About 1 mL of glycidol was placed in the reservoir nozzle and warmed to 35 °C. Liquid water was placed in a glass bulb which was then charged to 2 atm with a He-Ne mix (30:70). The He-Ne carrier gas, saturated with water vapor, was passed over the glycidol sample in the reservoir nozzle, just prior to expansion. The glycidol-H218O complex was formed using water-18O (95 atom %) obtained from Isotec. Deuterated complexes were formed by exchanging the hydroxyl hydrogen in deuterium oxide for 24 h, removing the solvent, and loading into the reservoir nozzle. He-Ne saturated with deuterium oxide was used as the carrier gas for the spectra of the deuterated complexes. Computational Method Model structures for the three glycidol conformations reported by Marstokk et al.22 were generated from HF/6-31G(d) optimizations using the Gaussian98 suite of programs29 on an R10000 workstation. The HF/6-31G(d) model structures reproduced both the rotational constants and relative energies reported by Marstokk. Structure optimizations of glycidol and glycidolwater were also conducted at the MP2 level using the 6-311++G(d,p) basis set. These calculations were carried out using Gaussian0330 on the Itanium-2 cluster at the Ohio Supercomputer Center. Vibrational frequencies were also calculated for the MP2 level structures to distinguish conformational minima from saddle points and to calculate the zero-point energy correction.
Glycidol. The rotational and centrifugal distortion constants of Marstokk22 were used to predict transition frequencies for the glycidol-I and glycidol-II conformers in the 11-20 GHz range. We used the 111-000 transition of conformer I (14129.787 MHz) and the 212-111 transition of conformer II (12618.138 MHz) as tune-up transitions. The constants in ref 22 predicted these transitions within 10 kHz (nearly experimental uncertainty). Spectral predictions for the two conformers were used to eliminate weak, high-J monomer transitions from the spectra of glycidol-water. We recorded rotational spectra for each of the three 13C isotopomers for both glycidol conformers I and II in natural abundance; the transition frequencies and fitting summaries for each 13C spectrum are provided in the Supporting Information. In each case, the rotational transitions were fit to rotational constants and a subset of the quartic centrifugal distortion constants using the Watson A-reduction Hamiltonian and the Ir representation.31 Some distortion constants (∆JK, ∆K, and δK) could not always be determined from the fits of limited data (9-17 transitions each), so these constants were fixed to the corresponding values for the normal isotopic species for each conformer. The best fit values of the rotational and centrifugal distortion constants for the six isotopic species are given in Table 1. The uncertainties given in the table represent 1 standard deviation. In all cases, the fit values of the distortion constants for the 13C isotopic species for the two conformers were in good agreement with the distortion constants obtained for the normal isotopic species.22 Spectral searches for transitions that could be assigned to glycidol conformer III were unsuccessful. We optimized the structure of glycidol both at the HF/631G(d) and MP2/6-311++G(d,p) levels. The results of the ab initio model structures are summarized in Table 2. The relative energies of the three conformers at the MP2 level retain the same ordering predicted by the HF level calculations, although the relative energies of conformers II and III are somewhat reduced compared to the HF predictions. Atomic coordinates are given in the Supporting Information for each of the model structures. Coordinates of the carbon and oxygen atoms show little difference (
