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Rotamers of o- and m-Dimethoxybenzenes Studied by Mass-Analyzed Threshold Ionization Spectroscopy and Theoretical Calculations† Shih Chang Yang,† Ssu Wei Huang, and Wen Bih Tzeng* Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, 1 Section 4, RooseVelt Road, Taipei 10617, Taiwan ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: August 3, 2010
We applied two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy to investigate the molecular properties of the selected rotamers of o-dimethoxybenzene (ODMB) and m-dimethoxybenzene (MDMB). The present experimental results show that only one stable configuration is involved in the photoexcitation and ionization processes of ODMB, as predicted by theoretical calculations. The adiabatic ionization energy (IE) of ODMB is measured to be 61 617 ( 5 cm-1. In the case of MDMB, both our experimental and calculated results suggest that there are three stable rotamers coexisting in the sample. The adiabatic IEs of rotamers a, b, and c are determined to be 63 523 ( 5, 64 491 ( 5, and 63 758 ( 5 cm-1. Analysis on the MATI spectra shows that most of the active cation vibrations of these isomeric species result from in-plane ring motions. In addition, different orientations of the two OCH3 groups have little effect on these vibrations. 1. Introduction Ion spectroscopy is an essential method to probe detailed properties of ionic species.1,2 Since its development, zero kinetic energy (ZEKE) photoelectron spectroscopy has been recognized as a powerful method for recording high-resolution spectra of molecular ions.3 This technique can give precise adiabatic ionization energy (IE) as well as spectroscopic information about internal motions of molecular ions. One of its important applications is to identify molecular conformers.4 With a similar experimental setup, mass-analyzed threshold ionization (MATI) spectroscopy5 involves detection of ZEKE ions rather than electrons. When the pulsed field sequence is carefully designed, MATI spectroscopy can provide unambiguous mass information with a comparable spectral resolution to that of ZEKE spectroscopy.6 It is known that a disubstituted benzene has various isomers that possesses different molecular properties.7 For example, there are three position (or structural) isomers of dihydroxybenzene: catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene). These isomers have very distinct chemical and physical properties and can be identified and isolated. However, each of these isomers may have rotational conformers (rotamers) resulting from different orientations of the hydrogen atoms of the two hydroxyl groups. Because the rotamers may coexist in chemical samples, the identification and isolation of a particular species can be quite challenging. Supersonic jet laser spectroscopy can be used to record vibronic spectra with discrete spectral features for each isomeric species.8 However, the origins of electronic transitions and vibrational frequencies of active mode for different rotamers may differ only as little as a few tens of wavenumbers. As a result, vibronic features of different isomeric species may overlap in a common spectral region.9 Confirmation of the †
Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Corresponding author. Fax: +886-2-23620200. E-mail:
[email protected]. † Current address: Taiwan International Graduate Program, Department of Chemistry, National Tsing Hua University and Academia Sinica.
existence of rotamers generally requires other complementary methods in addition to the typical spectroscopic approaches. Gerhards et al.10,11 applied MATI spectroscopy and theoretical calculations to investigate the rotamers of these dihydroxybenzenes. They concluded that catechol has only one stable structure, whereas resorcinol and hydroquinone have two rotamers. It is expected that the number of stable rotamers for each of ortho-, meta-, and para-dimethoxybenzenes may be different from those of dihydroxybenzenes. Because the OCH3 group is more bulky than the OH group, hydrogen bonding and steric hindrance should be taken into account when one considers the interaction between two substituents. Previous studies12-15 show that p-dimethoxybenzene (PDMB) has the two stable structures. Many molecular and spectroscopic properties of cis- or trans-PDMB have been reported. o-Dimethoxybenzene (ODMB) and m-dimethoxybenzene (MDMB) in the ground S0 and electronically excited S1 states were investigated by one-color resonant two-photon ionization (1C-R2PI), dispersed laser-induced fluorescence (LIF), and other spectroscopic methods.16-18 It was proposed that ODMB has only one stable form, whereas MDMB has three rotamers. Concerning the ionic properties, the IEs of ODMB and MDMB have been reported on the basis of photoelectron spectroscopy.16,19 Because of the experimental limitation arising from the low spectroscopic resolution, the molecular conformation of each species was not specified. To the best of our knowledge, the detailed spectroscopic data of the selected conformers of ODMB and MDMB in the cationic state are not yet available in the literature. In this Article, we report the vibrationally resolved cationic spectra of the selected rotamers of ODMB and MDMB by using the two-color resonant two-photon MATI technique. The species selection is achieved by tuning the frequency of the excitation laser to a particular vibronic level of the chosen isomer for successive excitation/ionization. To examine whether a significant change in molecular geometry during the electronic transition occurred and to obtain more information about the active cation vibrations, several vibronic states were used as the intermediate levels for recording the MATI spectra. The
10.1021/jp1026652 2010 American Chemical Society Published on Web 08/12/2010
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experimental results yield precise adiabatic IEs of the rotamers and vibrational features of the corresponding cations. These data can be used as fingerprints to identify the existence of rotamers. We have also performed ab initio and density functional theory (DFT) calculations to predict the electronic transition energies, vibrational frequencies, and many other molecular properties to support our experimental findings. Comparing these data with those of benzene, anisole, and other disubstituted derivatives helps us to learn more about the effects of substituent on the transition energy and molecular vibration. 2. Experimental and Computational Details 2.1. Experimental Method. All of the experiments reported here were performed with a laser-based time-of-flight mass spectrometer described elsewhere.20 ODMB (99% purity) and MDMB (98% purity) were purchased from Sigma-Aldrich and used without further purification. The vapors of these liquid samples kept at room temperature were seeded into 2 to 3 bar of helium and expanded into the vacuum through a pulsed valve with a 0.15 mm diameter orifice. During the experiments, the gas expansion and the ionization regions were maintained at a pressure of about 1 × 10-3 and 1 × 10-5 Pa, respectively. We initiated the two-color resonant two-photon excitation process by utilizing two independent tunable UV laser systems controlled by a delay/pulse generator (Stanford Research Systems, DG 535). The excitation (pump) source is a Nd/YAG (Quanta-Ray GCR-3) pumped dye laser (Lambda-Physik, Scanmate-2 OG with BBO-I crystal; rhodamin 575 and coumarin 540A dyes) with bandwidth e0.3 cm-1. The visible radiation is frequency doubled to produce UV radiation. The ionization (probe) UV laser (Lambda-Physik, Scanmate-2 with BBO-III crystal; LDS-772 and LDS-765 dyes) was pumped by a frequency-doubled Nd/YAG (Quanta-Ray LAB-150). A Fizeautype wavemeter (New Focus 7711) was used to calibrate the wavelengths of both lasers. These two counter-propagating laser beams were focused and intersected perpendicularly with the molecular beam at 50 mm downstream from the nozzle orifice. In the R2PI experiments, the total ion signal is collected. Analysis on the rising step in the photoionization efficiency (PIE) curve can yield an IE with an uncertainty of about 10-20 cm-1. In the MATI experiments, about 170 ns after the occurrence of the laser pulses, a pulsed electric field of -1.0 V/cm was switched on to reject the prompt ions. About 11 microseconds later, a second pulsed electric field of 140 V/cm was applied to field-ionize the Rydberg neutrals. These threshold ions were then accelerated and passed through a field-free region before being detected by a dual-stacked microchannel plate detector. The MATI technique leads to a sharp peak at the ionization threshold and thus gives a more precise IE value. In addition, it provides information about the active vibrations of the cation with a resolution better than those obtained from the conventional photoelectron experiments. The lowest energy band (labeled 0+) in a MATI spectrum corresponds to the signal of the vibrationless cations. Because the threshold ions are formed by pulsed field ionization (PFI) of high n Rydberg neutrals with energy of a few wave numbers below the true ionization threshold, the determination of the adiabatic IE should consider the high-energy side of the 0+ band.21,22 The energy lowering (in inverse centimeters) due to the Stark effect may be estimated by 4.0F1/2 when a pulsed field F (in volts per centimeter) is applied.23 This method generally leads to an uncertainty of 4-6 cm-1 in the determination of the IE. 2.2. Computational Method. We performed all ab initio and DFT calculations by using the Gaussian 03 package24 to obtain
Figure 1. Possible stable rotamers of ODMB predicted by the B3PW91/6-311++G** calculations. The trans form is the stable rotamer involved in the present experiment.
information about the molecular geometries, total energies, and vibrational frequencies of these molecules in the S0, S1, and D0 states. The IE was deduced from the difference in the zeropoint energies (ZPEs) of the cation and the corresponding neutral species in the ground state. Because frequency calculations are performed on the basis of the harmonic oscillator model, the obtained frequencies are scaled by an appropriate value to correct approximately for the combined errors stemming from basis set incompleteness, neglect of electron correlation, and vibrational anharmonicity.25 The calculated vibrational frequencies and scaling factors are listed in the Tables along with the measured values. 3. Results 3.1. Cation Spectra of ODMB. We have applied the 1CR2PI technique to record the vibronic spectrum of ODMB in the wavelength range near its S1 r S0 electronic origin. The vibronic spectrum is not shown because its major spectral features are nearly identical to those reported in the literature.17,18 Although ODMB may have many possible rotational isomers (rotamers), as shown in Figure 1, only the trans form is involved in the present experimental study. The detailed discussion is stated in Section 4.1. Figure 2 displays the MATI spectra of ODMB recorded by ionizing through the vibrationless 0° (35 750 cm-1) and 6a1 (0° + 506 cm-1) levels in the S1 state. In consideration for the uncertainty of the laser photon energy, spectral width, and the Stark effect,23 the adiabatic IE of ODMB is determined to be 61 617 ( 5 cm-1 (7.6395 ( 0.0006 eV), which is in good agreement with the reported value of 7.8 ( 0.15 eV.19 The assignment of this MATI spectrum of the ODMB cation in the D0 state was made by comparing with (1) the experimental data of this molecule in the S1 and S0 states,17,18,26 (2) the experimental data of the MDMB and PDMB15 cations, and (3) the calculated values. All observed MATI bands are listed in Table 1, along with the calculated values and their possible assignments. When the S10° state is used as intermediate level, many vibrations of cation appear to be active, as seen in Figure 2a. The intense bands at 458, 571, and 755 cm-1 result from the in-plane ring vibrations 6b1, 6a1, and 11, respectively. The substituent-sensitive in-plane C-OCH3 bending vibrations modes 15 and 9b appear at 186 and 350 cm-1. The vibrational frequencies of vibration 151 and 9b1 of o-methoxyphenol are 332 and 527 cm-1, respectively.27 It is clear that a heavier substituent gives rise to a smaller frequency for these substituentsensitive in-plane bending vibrations. The low-frequency bands at 110 and 225 cm-1 result from the out-of-plane O-CH3
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Figure 2. MATI spectra of ODMB, recorded by ionizing via the (a) 0° and (b) 6a1 levels in the S1 state.
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Figure 3. Some normal vibrations of ODMB. The open circles designate the original locations of the atoms, whereas the solid dots mark the displacements. The measured and calculated (in the parentheses) frequencies of each mode are listed in Table 1.
TABLE 1: Observed Bands (inverse centimeters) in MATI Spectra of ODMB and Possible Assignmentsa exptl
calcd
assignmentb
110 186 225 272 350 373 458 490 571 755 980 1008 1180 1316 1370
110 182
X1, γ(O-CH3) 151, β(C-OCH3), β(CCC) X2, γ(O-CH3) 10a1, γ(CCC) 9b1, β(C-OCH3) 152, β(C-OCH3), β(CCC) 6b1, β(CCC) 16b1, γ(CCC) 6a1, β(CCC) 11, breathing 1 1 X2 18b1, β(CH) 19a1, β(CH) 6a111 141, V(CCC)
273 363 442 489 569 752 1009 1171 1367
a Experimental values are whereas the predicted ones 6-311++G** calculation and stretching, in-plane, and respectively.
Figure 4. Stable rotamers of MDMB predicted by the B3PW91/6311++G** calculations.
shifts (cm-1) from 61 617 cm-1, are obtained from the B3PW91/ scaled by 0.98. b β and γ represent out-of-plane bending vibrations,
bending (designated as X) and its overtone vibrations. As seen in Figure 2b, when the MATI spectrum was recorded via 6a1 vibrational level in the S1 state, a strong band at 574 cm-1 corresponding to the in-plane bending 6a1 vibration is observed. This indicates that the molecular geometry and vibrational coordinates of ODMB cation are like those of the neutral species in the S1 state. The vibrational patterns of modes 1, 6a, 9b, and X are shown in Figure 3. 3.2. Vibronic and Cation Spectra of MDMB Rotamers. Our B3PW91/6-311++G** calculations predict that three stable rotamers of MDMB may exist, as shown in Figure 4. To confirm the existence of these rotamers, we have applied the 1C-R2PI, 2C-R2PI, and MATI techniques. Figure 5 shows the vibronic spectrum of jet-cooled MDMB in the energy range near its S1 r S0 electronic transition. The pronounced bands at 36 117 ( 2, 36 185 ( 2, and 36 268 ( 2 cm-1 are identified as the band origins of rotamers a, b, and c, respectively, which are consistent with those previously reported.17 However, previous work only focuses on the weak low-frequency bands related to methoxy
Figure 5. Vibronic spectrum of MDMB. The transition origins of rotamers a, b, and c appear at 36 117, 36 185, and 36 268 cm-1, respectively. All other labels correspond to the transitions mainly involving vibrations of rotamer a. The detailed assignments for the observed bands are listed in Table 2.
torsions. Here we concentrate on the transitions corresponding to the ring vibrations. Table 2 lists the observed bands, photon energy, relative intensity, calculated frequencies, and possible assignments. Because rotamer a is the most abundant species among the possible conformational isomers of MDMB, the intense bands in the vibronic spectrum are assigned to its in-
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TABLE 2: Observed Bands in the Vibronic Spectrum of MDMB and Their Possible Assignmentsa energy (cm-1)
relative intensity
36 117 36 303 36 351 36 397 36 426 36 454 36 485 36 541 36 605 36 658 36 798 36 809 36 836 36 938 37 072 37 114
100 19 28 15 14 6 10 11 5 51 42 79 19 5 23 8
Rotamer a 0 186 187 234 280 267 309 279 337 368 424 488 463 541 550 681 692 693 719 821 955 938 997 1000
0°0, band origin 9a10, β(C-OCH3) 10b20, γ(CCC) 1510, β(C-OCH3) 10b20, γ(O-CH3) 10a20, γ(CCC) 9a20, β(C-OCH3) 9a1010b20 6b10, β(CCC) 6a10, β(CCC) 9a106b10 110, breathing 9a106a10 15106a10 1210, β(CCC) 18a10, β(CH)
36 185 36 878 37 139
26 10 11
Rotamer b 0 693 693 954 941
0°0, band origin 110, breathing 1210, β(CCC)
36 268
8
Rotamer c 151
0°0, band origin
shift (cm-1)
calcd (cm-1)
assignment and approx. descriptionb
Figure 6. PIE curves of the rotamers a, b, and c of MDMB recorded by ionizing via their respective S101 levels.
a
Experimental values are shifts from 36 117, 36 185, and 36 268 cm-1, respectively, whereas the calculated ones (scaled by 0.90) are obtained from the CIS/6-311+G* calculations. b β, in-plane ring bending; γ, out-of-plane ring bending.
plane ring vibrations. The strong bands at 541, 692, and 955 cm-1 result from transitions 6a10, 110, and 1210, related to the in-plane ring deformation vibration of rotamer a, respectively. The low-frequency bands at 186 and 280 cm-1 result from transitions 9b10 and 1510, involving the substituent sensitive inplane bending vibration. Evidently, the 110 and 1210 transitions of rotamer b appear at 693 and 954 cm-1, respectively. Comparing these data with those of rotamer a, one may conclude that the orientation of the two OCH3 groups does not influence the frequency of these two in-plane ring deformation vibrations. The vertical value of the IE of MDMB has been reported to be 8.14 eV on the basis of photoelectron spectroscopy.16 The molecular conformation of MDMB was not specified because the method used lacks energy resolution. In this study, we have applied both PIE and MATI methods to measure the adiabatic IEs of the selected rotamers of MDMB. The former technique involves detection of the prompt ions, whereas the latter is subject to the detection of the threshold ions.28 Because the signal resulting from the prompt ions is generally stronger than that from the threshold ions, we have performed the PIE experiments to locate the ionization limit of selected rotamer before applying the MATI technique. Figure 6 shows the PIE curves of rotamers a, b, and c of MDMB by using the 2C-R2PI technique. Investigations on the rising steps give the respective adiabatic IEs of 63 517, 64 487, and 63 753 cm-1 with an uncertainty of ∼15 cm-1. Figure 7 displays the MATI spectra of rotamers a, b, and c of MDMB, recorded by ionizing via their respective S10° levels. Analysis on the sharp 0+ bands with consideration of the uncertainty in the laser photon energy, the spectral width, and the Stark effect gives the adiabatic IEs of 63 523 ( 5 (7.8759 ( 0.0006 eV), 64 491 ( 5 (7.9959 ( 0.0006 eV), and 63 758 ( 5 cm-1 (7.9050 ( 0.0006 eV) for MDMB-a, MDMB-b, and MDMBc, respectively. It is evident that the IEs of these rotamers
Figure 7. MATI spectra of the rotamers a, b, and c of MDMB recorded by ionizing via their respective S101 levels.
determined by the MATI technique are in excellent agreement with those measured by the PIE method. Both PIE and MATI experimental results confirm the existence of the conformational isomers of MDMB. Another advantage of the MATI over the PIE experiments is that it provides information about the internal motions of the cation.1,6,28 The spectral features on the high-energy side of the 0+ band correlate to the internal motions of cation. Because the rotational energy is smaller than the vibrational energy by about three orders of magnitude, the observed MATI bands in this Article are only considered to result from molecular vibrations in its cationic state. Table 3 lists the frequencies of the observed bands in Figure 7 and their possible assignments. The spectral assignment to these MATI features of the ODMB cation in the D0 state was made by comparing with (1) the experimental data of this molecule in the S0 state,26 (2) the experimental data of the ODMB, PDMB, m-methoxyphenol,4 and resorcinol,11 and (3) the calculated values. Most of the bands with frequency higher than 300 cm-1 arise from the active ring vibrations. The bands corresponding to the 11, 121, 18a1, and 141 vibrations appear at 715, 961, 1100, and 1352 cm-1 for rotamer a; 707, 960, 1103, and 1345 cm-1 for rotamer b; and 716, 965, 1100, and 1355 cm-1 for rotamer c, respectively. This shows that different configuration of the two methoxy groups
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TABLE 3: Observed Bands (inverse centimeters) in the MATI Spectra of the Rotamers a, b, and c of MDMB in Figure 7 and Their Assignmentsa MDMB-a exptl
182 272 452 485 572 715 961 1100 1352
calcd
177 262 451 487 569 712 958 1095 1345
MDMB-b exptl
538 707 960 1103 1345 1538
calcd
545 709 957 1107 1369 1531
MDMB-c exptl
calcd
59 128
96 133
716 965 1100 1355 1531
709 960 1094 1384 1546
assignment and approx. descriptionb γ(O-CH3) γ(O-CH3) 9a1, β(C-OCH3) 151, β(C-OCH3) 9a1151 6b1, β (CCC) 6a1, β (CCC) 11, breathing 121, β(CCC) 18a1, β(CH) 141, ν(CC) 8a1, ν(CC)
a Experimental values are shifts from 63 523, 64 491, and 63 758 cm-1, respectively, whereas the calculated ones (scaled by 0.98) are obtained from the B3PW91/6-311++G** calculations. b β, in-plane ring bending; γ, out-of-plane ring bending; ν, stretching.
Figure 9. MATI spectra of MDMB-a, recorded by ionizing via its (a) 6a1 and (b) 11 levels in the S1 state.
TABLE 4: Observed Bands (in inverse centimeters) in the MATI Spectra of MDMB-a in Figures 8 and 9 and Their Possible Assignmentsa intermediate level in the S1 state 9a01
10b2
151
6a1
182 211 271
182 214 278 324 366 428
182
184
362
11
calcd 177
270
262
365 426 570
569
594 657 722 757 897 992
Figure 8. MATI spectra of MDMB-a, recorded by ionizing via its (a) 9a1, (b) 10b2, and (c) 151 levels in the S1 state.
has little effect on the vibrational frequencies of these ring vibrations. Similar findings have also been found for resorcinol,11 hydroquinone, and p-dimethoxybenzene.15 Figures 8 and 9 display the MATI spectra of MDMB-a recorded by ionizing via the 9a10, 10b20, 15110, 6a10, and 110 vibrational levels in the S1 state. All of the frequencies of the observed bands and their approximation descriptions as well as the calculated values are listed in Table 4. Because these intermediate states mainly involve the in-plane ring vibrations of the neutral species in the S1 state, the strong bands in the MATI spectra are found to relate to the in-plane ring vibrations of cation. As depicted in the Figures, the most prominent bands at 182, 270, 570, and 718 cm-1 result from in-plane ring vibrations 9a1, 151, 6a1, and 11 of MDMB-a, respectively. As previously stated, frequencies of the corresponding modes of the neutral species in the S1 state are measured to be 186, 280, 541, and 692 cm-1, respectively. Evidently, the frequencies of these in-plane ring vibrations in the D0 state may deviate from those of the corresponding ones in the S1 state. The discrepancy in the frequencies of the ring vibrations might arise from the change in the electronic density near the aromatic ring of the molecule upon the D0 r S1 transition. When the out-of-plane vibration S110b2 is used as intermediate level, bands at 214 and
718
712 885
980
assignment and approx. descriptionb 9a1, β(C-OCH3) 10b2, γ(CCC) 151, β(C-OCH3) 10b2 γ(O-CH3) 9a2 or 16b1, γ(CCC) 10b4, γ(CCC) 6a1, β(CCC) 10b49a1 10b6, γ(CCC) 11, breathing 9a16a1 7b1 ν(C-OCH3) 15111
a Experimental values are shifts from 63 523 cm-1, whereas the calculated values (scaled by 0.98) are obtained from the B3PW91/ 6-311++G** calculations. b β, in-plane ring bending; γ, out-of-plane ring bending; ν, stretching.
428 cm-1 also correspond to 10b2 and 10b4 overtone vibrations, respectively. 4. Discussion 4.1. ODMB. Bernstein and coworkers17 pointed out that ODMB may have as many as 12 rotational conformers resulting from relative orientation of the two methoxy groups. However, only one stable rotamer is involved in their spectroscopic experiments. Houk and coworkers16 performed ab initio calculation with the STO-3G basis set. They reported that ODMB has a preferred nonplanar conformation based on their results of the photoelectron spectroscopic experiments and calculations. To investigate further the structure of ODMB, Pratt and coworkers18 recorded the rotationally resolved S1 r S0 excitation fluorescence spectra of ODMB and ODMB-H2O complex. They concluded that the stable structures of ODMB in both S0 and S1 states have a trans form where the oxygen atoms of two methoxy groups lies in the same plane as the aromatic ring. In the present study, we performed DFT calculations to search systematically for possible stable rotamers of ODMB. The hybrid Becke three-parameter with the PW91 correlation functional (B3PW91) calculations with the 6-311++G** basis set show that the trans and gauche forms are stable in the S0
Rotamers of o- and m-Dimethoxybenzenes state. The ZPEs of trans and gauche rotamer are predicted to be -461.068141 and -461.066057 hartree, respectively. Therefore, the gauche form lies higher than that of the trans rotamer by 457 cm-1. In the studies of rotational conformers of n-propylbenzene and n-p-propylaniline, Reilley, Kimura, and their coworkers29,30 assumed that internal conversion of rotamers is very slow under supersonic expansion. They further proposed that the ratio of the band intensities of the gauche and trans rotamers (Ig/It) might reflect their relative abundances at the nozzle temperature. A simple relationship is expressed as follows, Ig/It ) exp(-∆E/kT), where ∆E is the relative energy between the two rotamers and T is the nozzle temperature in kelvin. When making a similar assumption, we obtain Ig/It ) 0.12 at a nozzle temperature of 298 K. This indicates that the signal from the gauche rotamer may be too low to be detected in our experiments. This result is inconsistent with those reported by Bernstein and Pratt and their coworkers.17,18 Similarly, only one stable rotamer is observed in the spectroscopic experiments of catechol,10 o-fluorophenol,27 and o-methoxyphenol.27 Both in-plane ring deformation vibration 6a and breathing mode 1 appear to be active in our vibronic and MATI spectra of ODMB. The respective frequencies are 506 and 728 cm-1 in the S1 state and 571 and 755 cm-1 in the D0 state. Although there is no experimental data for mode 1, the frequency of vibration 6a of ODMB in the S0 state was reported to be 580 cm-1.26 This finding can be understood as follows. The electron density nearby the aromatic ring of ODMB changes upon the S1 r S0 and D0 r S1 electronic transitions. It follows that the force constant of the neutral species in the S1 state is less than that of the neutral species in the S0 state or the cation in the D0 state. Because frequency is proportional to the square root of the force constant, one expects a slightly less vibrational frequency in the S1 state, as observed in the present experiments. The measured frequencies of the corresponding vibrations of the rotamers of MDMB in the S1 and D0 states are listed in Tables 2 and 3. These data also give a similar suggestion that molecular geometry in the S1 state is slightly looser than those in the S0 state or the D0 state. 4.2. MDMB. Similar to ODMB, many possible conformers of MDMB exist because of different orientations the two methoxy substituents. Bernstein and coworkers17 applied 1CR2PI and dispersed emission techniques to study the stable conformation of MDMB. They concluded that there are three different rotamers involved in their supersonic beam experiments. In the present study, our B3PW91/6-311++G** calculations predict that MDMB has three stable rotamers with the patterns shown in Figure 4. The ZPEs of rotamers a, b, and c in the S0 state are calculated to be -461.074486, -461.073827, and -461.073292 hartree, respectively. This result shows that rotamer a is the most stable form, whereas rotamers b and c lie in higher energy levels of 155 and 250 cm-1, respectively. Following the assumption proposed by Reilley, Kimura, and their coworkers,29,30 one expects that the intensity ratio of the band origins of rotamers a, b, and c is 1.00:0.48:0.30. The observed intensity ratio in Figure 5 is 1.00:0.24:0.09. The discrepancy between the predicted and the observed intensity ratio of isomers may be due to the following reasons. Upon electronic excitation, various rotamers may demonstrate different degrees of change in geometry. This gives rise to different Franck-Condon factors for various rotamers. Second, the electronic temperature may not be the same as the nozzle temperature. The detailed consideration on the discrepancy between the prediction and observation is beyond the scope of this Article.
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11149 In the present study, we applied PIE and MATI experiments in combination with two-photon excitation. As previously described, the results of our PIE and MATI experiments clearly prove the existence of three rotamers for MDMB. The adiabatic IEs of rotamers a, b, and c were determined to be 63 523, 64 491, and 63 758 cm-1, respectively. Furthermore, each rotamer has its unique cation spectrum, as shown in Figure 7. Therefore, our experimental and calculation findings support the conclusion drawn by Bernstein and coworkers.17 4.3. Vicinal Substitution Effect. It is known that the functional group of benzene derivatives can interact with ring by the inductive effect through the σ bond or by the conjugation effect through the π orbitals. The collective effect results in a slight change of the nearby electron density and molecular geometry. It causes a lowering of the ZPE of the electronic state. If the magnitude of lowering of the ZPE for the upper state is greater than that of lower one, then it yields a red shift in the transition energy. On the contrary, it gives a blue shift. Table 5 lists the electronic excitation and ionization energies of benzene, phenol, anisole, and their derivatives on the basis of the LIF, R2PI, ZEKE, or MATI experiments.4,10,11,15,27,31-42 Analyses on these data indicate that both OH and OCH3 substituents cause red shifts in the electronic excitation and IE. The red shifts of the S1 r S0 transition energy are 2437, 2174, and 2336 cm-1 for catechol, o-methoxyphenol, and o-dimethoxybenzene, respectively, whereas the IEs are 8679, 10 562, and 12 940 cm-1, respectively. Comparison on the data finds that the OH group yields a slightly greater shift in the S1 r S0 excitation energy (E1) and a much less red shift in the D0 r S1 transition energy than the OCH3 group. Our previous studies15,31 indicate that the S1 r S0 excitation causes an expansion in the aromatic ring, and the D0 r S1 transition is subject to the removal of an electron nearby the oxygen atom. With consideration of the redistribution of charge, the electron density near oxygen is expected to increase by the contribution of the electrons of the CH3 group through the σ bond. Therefore, the OCH3 substitution will lower the IE more than the OH substitution, as seen in Table 5. As consequence of CH3 group stabilization, the OCH3 group causes a greater red shift in the IE than the OH group. Similar trends can also be found when the OH and OCH3 groups are at meta and para positions. Another observation is that the IEs of structural isomers of different substituted aromatic molecule follow the order para < ortho < meta for these species. As shown in Table 5, the E1 values of anisole32 and ODMB are 36 383 and 35 750 cm-1, respectively. The corresponding ones of MDMB-a, MDMB-b, MDMB-c, are determined to be 36 117, 36 185, and 38 268 cm-1, respectively, and the respective values of cis- and trans- PDMB15 are 33 852 and 33 631 cm-1. In other words, an additional methoxy group leads to a greater red shift by 633 cm-1 for ODMB, 266, 198, and 115 cm-1 for MDMB, and 2531 and 2752 cm-1 for PDMB. This indicates that the red shift in the E1 values of dimethoxybenzenes follows the order: meta < ortho < para. As previously mentioned, the S1 r S0 electronic excitation causes an expansion of the aromatic ring, leading to a stronger interaction between substituent and the ring in the S1 state. As a result, it yields a red shift in the E1. Huang et. al33,34 pointed out that phenol, aniline, and its p-fluoro-substituted derivatives shows quinoid-like resonance structure in the first excited state on the basis of their Stark effect experiments. The experimental results show that the interaction between the aromatic ring and the substituents is the greatest when two OCH3 groups are at the para position. Similar results are also found for the o-, m-, and p-fluorophenol.27,35,36
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TABLE 5: Measured Transition Energies (in inverse centimeters) of Benzene and Its Derivativesa molecule b,c
benzene phenold anisolee catecholf resorcinol, cisg resorcinol, transg hydroquinone, cish hydroquinone, transh o-methoxyphenoli m-methoxyphenol, aj m-methoxyphenol, bj m-methoxyphenol, cj p-methoxyphenol, cisk p-methoxyphenol, transk o-dimethoxybenzenel m-dimethoxybenzene, al m-dimethoxybenzene, bl m-dimethoxybenzene, cl p-dimethoxybenzene, cish p-dimethoxybenzene, transh g
S 1 r S0
∆E1
D0 r S 1
∆E2
IE
∆IE
38 086 36 349 36 383 35 649 36 196 35 944 33 535 33 500 35 912 35 974 36 034 36 202 33 667 33 572 35 750 36 117 36 185 36 268 33 852 33 631
0 -1737 -1703 -2437 -1890 -2142 -4551 -4586 -2174 -2112 -2052 -1884 -4419 -4514 -2336 -1969 -1901 -1818 -4234 -4455
36 471 32 276 30 016 30 229 30 952 30 751 30 516 30 498 28 083 29 254 28 707 29 446 28 646 28 638 25 867 27 406 28 306 27 490 26 920 26 932
0 -4195 -6455 -6242 -5519 -5720 -5955 -5973 -8388 -7217 -7764 -7025 -7825 -7833 -10 604 -9065 -8165 -8981 -9551 -9539
74 557 68 625 66 399 65 878 67 148 66 695 64 051 63 998 63 995 65 228 64 741 65 648 62 313 62 210 61 617 63 523 64 491 63 758 60 772 60 563
0 -5932 -8158 -8679 -7409 -7862 -10 506 -10 559 -10 562 -9329 -9816 -8909 -12 244 -12 347 -12 940 -11 034 -10 066 -10 799 -13 785 -13 994
a ∆E1, ∆E2, and ∆IE are shifts of S1 r S0, D0 r S1, and IE with respect to those of benzene. b Ref 37. c Ref 38. d Ref 39. e Ref 32. f Ref 10. Ref 11. h Ref 15. i Ref 27. j Ref 4. k Ref 40. l This work.
Houk and coworkers16 in the photoelectron spectroscopic experiments determined the IEs of ODMB, MDMB and PDMB to be 8.17, 8.14, and 7.96 eV, respectively. These values represent the vertical IEs and are not specified for the isomers. In contrast, our resonant two-photon MATI experiments give precise adiabatic IEs with the uncertainty of ∼5 cm-1 and cation spectra of the selected rotamers of ODMB, MDMB, and PDMB. The adiabatic IEs are determined to be 61 617 cm-1 for ODMB; 63523, 64491, and 63758 cm-1 for rotamers a, b, and c of MDMB, and 607 72 and 60 563 cm-1 for cis and trans rotamers of PDMB,15 respectively. Therefore, the IEs of the position isomers of dimethoxybenzene follow the order: PDMB < ODMB < MDMB. Similar results are also reported for the position isomers of dihydroxybenzene and methoxyphenol.4,10,11,15,27,40-42 Vibrations 6b, 6a, and 1 are found to be active for the position isomers of dimethoxybenzene. The cation vibrational frequencies of modes 6b, 6a, and 1 appear at 458, 571, and 755 cm-1 for ODMB; 488, 541, and 692 cm-1 for MDMB-a; and 540, 395, and 816 cm-1 for trans-PDMB,15 respectively. The corresponding frequencies of vibrations 6a and 1 of the cis-PDMB cation are reported to be 523 and 818 cm-1,15 respectively. This suggests that the frequencies of the in-plane ring vibrations depend on the relative location of two methoxy groups on the aromatic ring as well as the vibrational pattern. 4.4. Rotational Barrier of Isomerization. In accordance with previous findings,17 there are three stable rotamers of MDMB observed in our R2PI and MATI experiments. One may ask whether isomerization takes place upon electronic transition. To answer this question, we performed the conformational energy analyses, which are implemented in the B3PW91/6311++G** calculations. The dihedral angles θ1 and θ2 define the C-O-CH3 bond with respect to the plane of the aromatic ring for the two methoxy groups. One-dimensional potential energy surface calculations were carried out at θ1 ) 0 and 180°, whereas θ2 varied from 0 to 360° with an increment of 10° for each step. Figure 10 shows the 1D potential energy surfaces for the conversion between rotamers a and b in the S0 and D0 states, whereas Figure 11 is for the conversion between rotamers a and c. These results predict that the energy barrier for isomerization between MDMB-a and MDMB-b in the S0 state is in
Figure 10. One-dimensional potential energy surfaces for the conversion between rotamers a and b of MDMB in the (a) D0 and (b) S0 states.
the range of 1089-1244 cm-1 and that between MDMB-a and MDMB-c is 978-1225 cm-1. In the D0 state, the isomerization barriers for these rotamers are greatly enhanced by a factor of 2 to 3. This is due to the fact that the interaction between the OCH3 group and the ring is stronger in the D0 state than in the S0 state. The present calculated results suggest that the isomerization among rotamers of MDMB does not take place upon photoexcitation and ionization. Similar finding has been reported for m-methoxyaniline.42 5. Conclusions We have applied the two-color resonant two-photon MATI technique to record vibrationally resolved spectra of ODMB and MDMB in their cationic state. Similar to catechol,10 ODMB has only one stable configuration whose adiabatic IE is determined to be 61 617 ( 5 cm-1. However, there are three stable rotamers of MDMB found in the photoexcitation and ionization processes in our experiments. The adiabatic IEs of
Rotamers of o- and m-Dimethoxybenzenes
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11151 References and Notes
Figure 11. One-dimensional potential energy surfaces for the conversion between rotamers rotamer a and c of MDMB in the (a) D0 and (b) S0 states.
MDMB are measured to be 63 523 ( 5, 64 491 ( 5, and 63 758 ( 5 cm-1, respectively. Comparing this result with that of resorcinol, one clearly finds that MDMB has one more stable rotamer than resorcinol. Table 5 shows that the degree of the red shift of the IEs of MDMB is greater than that of resorcinol by ∼50%. It implies that the interaction between the OCH3 substituent and the aromatic ring is greater than that of the OH group and the ring. This may account for the fact that MDMB has one more stable rotamer than resorcinol. Our theoretical calculations predict that the isomerization among rotamers of MDMB does not take place upon photoexcitation and ionization, as was reported for m-methoxyaniline rotamers.42 Comparing the IEs of ODMB and MDMB with that reported for PDMB,15 one finds that the IEs of the position isomers of dimethoxybenzene follow the order: para < ortho < meta. Similar results have been reported for the position isomers of dihydroxybenzene,10,11,15 methoxyphenol,4,27,40 methoxyaniline,41,42 and fluorophenol.27,35,36 Most of the observed active vibrations of cations of the selected rotamers of ODMB and MDMB result from the inplane ring vibrations. In general, the molecular geometry and vibrational coordinates of the cation are like those of the neutral species in the S1 state. Detailed analysis of the MATI spectra shows that spectral features are quite similar among the three rotamers of MDMB. The vibrational frequencies of modes 1, 12, 18a, and 14 are measured to be 715, 961, 1100, and 1352 cm-1 for rotamer a; 707, 960, 1103, and 1345 cm-1 for rotamer b; and 716, 965, 1100, and 1355 cm-1 for rotamer c, respectively. The results suggest that different orientation of two methoxy groups has little effect on the in-plane ring vibration. Vibrations 6b, 6a, and 1 of o-, m-, and p-dimethoxybenzene cations appear to be active. The cation vibrational frequencies of modes 6b, 6a, and 1 appear at 458, 571, and 755 cm-1 for ODMB; 488, 541, and 692 cm-1 for MDMB-a; and 540, 395, and 816 cm-1 for trans PDMB, respectively. This shows that frequencies of these in-plane ring vibrations depend on the relative location of two methoxy groups on the aromatic ring as well as the vibrational pattern. Acknowledgment. We gratefully acknowledge financial support from the National Science Council of the Republic of China under grant number NSC-98-2113-M-001-023-MY3.
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