A new method for investigating infrared spectra of protonated benzene (C6H7 +) and
cyclohexadienyl radical (c-C6H7) using para-hydrogen
Mohammed Bahou, Yu-Jong Wu, and Yuan-Pern Lee
Citation: The Journal of Chemical Physics 136, 154304 (2012); doi: 10.1063/1.3703502 View online: http://dx.doi.org/10.1063/1.3703502
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/136/15?ver=pdfcov Published by the AIP Publishing
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A new method for investigating infrared spectra of protonated benzene
(C
6H
7+) and cyclohexadienyl radical (
c
-C
6H
7) using
para
-hydrogen
Mohammed Bahou,1Yu-Jong Wu,2,a)and Yuan-Pern Lee1,3,a)
1Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University,
1001, Ta-Hsueh Road, Hsinchu 30010, Taiwan
2National Synchrotron Radiation Research Center, 101, Hsin-Ann Road, Hsinchu 30076, Taiwan 3Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
(Received 21 February 2012; accepted 28 March 2012; published online 17 April 2012)
We use protonated benzene (C6H7+) and cyclohexadienyl radical (c-C6H7) to demonstrate a new
method that has some advantages over other methods currently used. C6H7+ and c-C6H7were
pro-duced on electron bombardment of a mixture of benzene (C6H6) and para-hydrogen during
deposi-tion onto a target at 3.2 K. Infrared (IR) absorpdeposi-tion lines of C6H7+decreased in intensity when the
matrix was irradiated at 365 nm or maintained in the dark for an extended period, whereas those of c-C6H7increased in intensity. Observed vibrational wavenumbers, relative IR intensities, and
deu-terium isotopic shifts agree with those predicted theoretically. This method, providing a wide spectral coverage with narrow lines and accurate relative IR intensities, can be applied to larger protonated polyaromatic hydrocarbons and their neutral species which are difficult to study with other methods. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3703502]
I. INTRODUCTION
Protonated aromatic hydrocarbon molecules are of fun-damental importance to organic chemistry as their existence has been proposed in diverse reactions. They are widely ac-cepted as important intermediates (Wheland intermediate) in electrophilic aromatic substitution reactions.1 Protonated
polycyclic aromatic hydrocarbons (PAH) are also postulated to be the carriers of the unidentified infrared emission (UIE) bands observed in various interstellar media.2In addition, the
effect of protonation on aromatic biomolecules is an interest-ing issue for models rationalizinterest-ing the ultraviolet (UV) photo-stability of biological molecules such as proteins and DNA.3,4
Protonated benzene (C6H7+), the simplest protonated
aromatic hydrocarbon, might exist as three conformers: the σ-complex (1), the bridged π -complex (2), and the face-centered π -complex (3), as shown in Fig.1. NMR spectra of C6H7+in superacid indicate that the most stable conformer of
C6H7+at low temperature is the σ -complex (1) having a C2v
planar structure.5–8The experimental proton affinity of C6H6
fits well with the theoretical value for the formation of (1), supporting that C6H7+ exists as a σ -complex.9–12
Quantum-chemical computations also support this assignment and pre-dict that the bridged π -complex (2) is a transition state for a 1,2-H shift within (1) and that the face-centered π complex (3) is a second-order saddle point.13,14
Early low resolution UV (Refs.15and16) and infrared (IR) (Refs.17and18) spectral data of C6H7+isolated in
su-peracid solutions or matrices were perturbed by the acid. The electronic absorption due to the transition A 1B
2 ← X 1A1
of C6H7+ in solid Ne was reported near 325 nm.19 For IR
spectra of gaseous C6H7+, two methods have been employed.
a)Authors to whom correspondence should be addressed. Electronic
addresses: yjwu@nsrrc.org.tw and yplee@mail.nctu.edu.tw.
Solcà and Dopfer employed IR photodissociation (IRPD) and mass-selected ion detection to record the IR absorption in the C–H stretching region of C6H7+ tagged with inert ligands
such as Ar, N2, CH4, and H2O.20,21 Jones et al. produced
and stored C6H7+ in an ion-cyclotron-resonance ion trap to
obtain its IR multiphoton dissociation (IRMPD) spectra us-ing a free-electron laser and reported IR features of C6H7+at
1228 and 1433 cm−1,22as shown in Fig.2(a). Douberly et al.
recorded the IRPD spectrum of the Ar-tagged C6H7+
com-plex (C6H7+-Ar) in the spectral region 750−3400 cm−1with
numerous additional IR features,23as shown in Fig.2(b). The
bands observed with IRMPD (Ref.22) are much broader than those recorded with IRPD and the positions are redshifted be-cause of the anharmonic effect in multiphoton absorption. The IR spectrum of C6H7+-Ar is expected to be similar to that
of C6H7+ because of the weak interaction between C6H7+
and the ligands. The wavenumbers observed for C6H7+-Ar
with IRPD are in satisfactory agreement with those calculated quantum-chemically for C6H7+,23,24as compared in Fig.2(c),
but the observed intensities in the C−H stretching region are much greater than predicted. This is partly because the re-ported spectrum is an action spectrum recorded by monitor-ing the intensity of C6H7+ on tuning the wavelength of the
IR laser and the observed intensity was not scaled with the IR laser power, which increases with laser wavenumber in the region 800−3200 cm−1. Furthermore, the photodissociation
efficiency for C6H7+-Ar to form C6H7+ and Ar is expected
to increase with the IR excitation energy. Hence the reported action spectrum for dissociation of C6H7+-Ar shows features
in the C−H stretching region with intensities much more en-hanced than those from theoretical predictions.23
Neutralization of C6H7+ produces the cyclohexadienyl
radical (c-C6H7) and its isomers.19 The c-C6H7 radical is an
important intermediate in the initial step in the hydrogena-tion of aromatic compounds in both the gaseous phase and 0021-9606/2012/136(15)/154304/8/$30.00 136, 154304-1 © 2012 American Institute of Physics
154304-2 Bahou, Wu, and Lee J. Chem. Phys. 136, 154304 (2012)
FIG. 1. Possible structures of protonated benzene. (1) σ -complex, (2) bridged π -complex, and (3) face-centered π -complex.
the condensed phase. The UV-visible spectra of c-C6H7have
been studied in the condensed phase25–30 and in the gaseous phase.31–33 The dispersed laser-induced fluorescence spec-tra of c-C6H7 radical, produced via H-abstraction of 1,
4-cyclohexadiene by Cl atom, yield wavenumbers for six vibra-tional modes of the ground electronic state.34However, except
for the ν10 mode near 981 cm−1, these modes do not match
with those identified for c-C6H7 according to IR absorption
lines of a C6H6/Xe matrix that was irradiated by fast electrons
followed by annealing at 45 K.35Hence, it is important to
ob-tain an IR spectrum of C6H7 with improved signal-to-noise
ratio and spectral resolution.
The quantum solid para-hydrogen (p-H2) has emerged as
a unique host for matrix isolation spectroscopy.36–38 The
ex-tremely narrow spectral width and a diminished matrix cage effect are some of the unique properties of the p-H2 matrix.
We have demonstrated that free radicals that are difficult to be produced via photolysis in situ or photo-induced bimolec-ular reactions using conventional noble-gas matrices can be readily produced in a p-H2matrix.39–41Here we report a new
application of p-H2matrix isolation technique to produce
pro-tonated aromatic hydrocarbons and their neutral species. We
FIG. 2. Comparison of IR spectra obtained on (a) IR multiphoton dis-sociation of C6H7+ (Ref. 22), (b) IR photodissociation of C6H7+-Ar
(Ref. 23), (c) theoretical prediction for C6H7+ based on harmonic
vibra-tional wavenumbers calculated with the CCSD(T*)-F12a/VDZ-F12 method and anharmonic contributions and IR intensities calculated with the B2PLYP-D/VTZ method,24and (d) IR absorption of C
6H6/p-H2(1/1000) matrix
sam-ple with electron bombardment during deposition; lines due to C6H6 and c-C6H7are stripped.
use the IR spectra of C6H7+ and c-C6H7 to demonstrate the
advantages of this method. II. EXPERIMENTAL
In our experiments, a gold-plated copper plate cooled to 3.2 K served as both a cold substrate for the matrix sample and a mirror to reflect the incident IR beam to the detector.42,43 The cooling of the substrate was achieved with a Janis RDK-415 closed-cycle helium cryostat system. IR absorption spec-tra were recorded with a Fourier-spec-transform infrared spectrom-eter (Bomem, DA8) equipped with a KBr beamsplitter and a Hg-Cd-Te detector (cooled to 77 K) to cover the spectral range 450−5000 cm−1. Six hundred scans at a resolution
of 0.25 cm−1 were generally recorded at each stage of the experiment.
The C6H7+cation is produced by electron bombardment
of a gaseous sample of p-H2 containing a small proportion
of C6H6 during deposition. Typically, a gaseous mixture of
C6H6/p-H2 (1/1000–1/3000, 1.3 mmol h−1) was deposited
over a period of 3−5 h. An electron gun (Kimball Physics, Model EFG-7) was used to generate an electron beam with energy of 250 eV and beam current of 70 μA during the de-position period. The following mechanism for the production of C6H7+with an excess of p-H2was proposed.20
H2+ e−→ H2++ 2e−, (1)
H2++ H2 → H3++ H H= −36.0 kcal mol−1,
(2) C6H6+ H3+→ C6H7++ H2 H= −82.0 kcal mol−1.
(3) The listed enthalpies of reaction were calculated with the B3PW91/6-311++G(2d,2p) method. Electron impact ioniza-tion of H2 produces H2+, subsequent rapid exothermic
pro-ton transfer reactions (2)and(3) produce C6H7+. The
reac-tion of C6H6with H and the neutralization reaction of C6H7+
produce C6H7.
Normal H2 (99.9999%, Scott Specialty Gases) was
passed through a trap at 77 K before entering the p-H2
con-verter that comprised a copper cell filled with iron (III) ox-ide catalyst (Aldrich) and cooled with a closed-cycle refrig-erator (Advanced Research Systems, DE204AF). The effi-ciency of conversion was controlled by the temperature of the catalyst—typically approximately 13 K which gives a propor-tion of o-H2of less than 100 ppm. C6H6(99.8%, Aldrich) and
C6D6 (isotopic purity∼99%, Cambridge Isotope
Laborato-ries) were used without further purification. III. THEORETICAL CALCULATIONS
The energies, equilibrium structures, vibrational wavenumbers, and IR intensities were calculated using the GAUSSIAN09 program.44 Density-functional theory for
cal-culations were performed using the B3PW91 method which uses Becke’s three-parameter hybrid exchange functionals,45
and a correlation functional of Perdew and Wang exchange functional46 were conducted. The standard basis set, 6-311++G(2d,2p), was used. Analytic first derivatives were utilized in geometry optimization, and anharmonic vibrational wavenumbers were calculated analytically at each stationary point. Geometric parameters of c-C6H7and C6H7+predicted
with various methods, vibrational displacement vectors of C6H7+ predicted with the B3PW91/6-311++G(2d,2p)
method are available from the supplementary material.47
IV. RESULTS
A partial IR spectra of a C6H6/p-H2 (1/1000) matrix
and a C6H6/p-H2 (1/1000) matrix bombarded with an e-gun
emitting electrons with energy of 250 eV during deposition are shown in Figs.3(a) and3(b), respectively, for the spec-tral ranges 750−1320 cm−1. Many new features appear in
Fig.3(b). To differentiate these features, we performed sec-ondary photolysis at various wavelengths and identified lines in each group according to their correlations in intensity variations. The resultant difference spectrum upon photoly-sis at 365 nm for 2 h with a light-emitting diode is shown in Fig. 3(c). The difference spectrum was obtained on sub-traction of the spectrum recorded before irradiation from that recorded after irradiation; lines pointing upward indicate pro-duction, whereas those pointing downward indicate destruc-tion. Irradiation of the matrix at 365 nm is expected to release electrons trapped in the matrix and to neutralize the cations. In some experiments we kept the matrix in the dark for 8−12 h and observed a difference spectrum similar to that recorded upon 365-nm irradiation but with smaller changes
FIG. 3. Partial IR absorption spectra of matrix samples in regions 750−1320 cm−1. (a) C6H6/p-H2(1/1000) deposited at 3.2 K, (b) C6H6
/p-H2 (1/1000) with electron bombardment during deposition at 3.2 K,
(c) difference spectrum of the sample in (b) upon irradiation at 365 nm for 2 h; the intense absorption line of C6H6near 1037 cm−1has been
re-moved, (d) stick spectrum of C6H7+ simulated based on harmonic
vibra-tional wavenumbers calculated with the CCSD(T*)-F12a/VDZ-F12 method and anharmonic contributions and IR intensities calculated with the B2PLYP-D/VTZ method,24and (e) stick spectrum of C
6H7 simulated based on
an-harmonic vibrational wavenumbers and IR intensities calculated with the B3PW91/6-311++G(2d,2p) method.
TABLE I. Comparison of experimental vibrational wavenumbers (in cm−1) of c-C6H7.
Mode Symmetry LIF/gas IR/Xe IR/p-H2
ν1 a1 3080.4 (13)a ν2 a1 3030.5 (3) ν3 a1 3050.0 (10) ν4 a1 2768 2780.1 (23) ν5 a1 1571 ν6 a1 1425.6 (5) ν7 a1 1387 1394.7 (9) ν8 a1 1174 ν10 a1 981 958 959.8 (5) ν11 a1 864.1 (3) ν12 a1 559 ν16 a2 375 ν17 b1 2757.4 (10) ν18 b1 970.5 (1) ν19 b1 908 910.7 (7) ν21 b1 618/620 622.0 (100) ν22 b1 546 510.2 (11) ν24 b2 3056.8? (14)b ν27 b2 1389.7 (1) ν29 b2 1287 1289.9 (4) ν33 b2 600
Reference 34 35 This work
aRelative IR intensities normalized to the most intense line (ν
21) of c-C6H7are listed in
parentheses.
bThe ? mark indicates that the assignment is tentative.
in line intensities; presumably some electrons can recombine with the cations slowly in the dark.
As shown in Fig.3(c), lines at 864.1, 910.7, 959.8, 970.5, and 1289.9 cm−1increased in intensity upon UV irradiation. These features, demonstrating a correlated change in intensity at various stages of experiments and in separate experiments, are designated as group A. A complete list of lines in group A is shown in Table I. These lines might be assigned to ab-sorption of cyclohexadienyl (c-C6H7) radical because a
ma-jority of intense lines observed in this work correspond well with those reported previously for c-C6H7 in a Xe matrix, as
compared in TableI.35 The broad feature near 950 cm−1 in
Fig.3(c)is an artifact from the subtraction.
The features pointing downward at 819.3, 893.7, 987.6, 1047.5, 1075.5, 1184.8, 1187.6, and 1225.5 cm−1, shown in Fig. 3(c), also demonstrate a correlated change in intensity at various stages of experiments and in separate experiments. These features are designated as group B and assigned to C6H7+, as discussed in Sec.V B; a complete list is
summa-rized in TableII.
Experiments on electron bombardment of a mixture of C6D6/p-H2 (1/1000) were performed to provide more
infor-mation on the assignments of IR features in groups A and B. Similar experimental procedures were followed, and a repre-sentative difference IR spectrum in ranges of 700−1500 and 2650−2840 cm−1 upon irradiation at 365 nm is depicted in Fig.4(a). Lines indicated with * are due to residues from sub-traction of intense lines of C6D6. The features pointing
up-ward and indicated with wavenumbers 748.2, 758.1, 797.6, 831.3, 1237.8, 1246.1, 1247.7, and 2784.1 cm−1 belong to group A. A complete list of lines in group A observed in the
154304-4 Bahou, Wu, and Lee J. Chem. Phys. 136, 154304 (2012)
TABLE II. Wavenumbers (in cm−1) and IR intensities of experimental re-sults compared with theoretical predictions for C6H7+and C6H7+-Ar.
C6H7+ C6H7+-Ar
Modea Sym. Calculationb IRMPDc p-H2d IRPDc ν1 a1 3081 (1)e ν24 b2 3105 (4) 3107 ν2 a1 3083 (3) 3078 ν25 b2 3067 (2) ν4 a1 2813 (35) 2813.1 (22)e 2820 ν17 b1 2808 (13) 2798.5 (11) 2809 ν5 a1 1590 (36) 1603.4 (27) 1607 ν27 b2 1452 (100) 1433 1451.9 (100) 1456 ν6 a1 1446 (13) 1445.2 (8) ν29 b2 1338 (8) 1328.1? (7) 1334 ν7 a1 1239 (58) 1228 1225.5 (55) 1239 ν8 a1 1188 (12) 1187.6 (21) 1198 ν30 b2 1185 (8) 1184.8 (10) ν31 b2 1122 (1) 1075.5? (4) ν18 b1 1045 (2) 1047.5? (1) 1058 ν32 b2 960 (9) 987.6 (6) 964 ν11 a1 888 (8) 893.7 (10) 903 ν20 b1 820 (9) 819.3 (9) 831 ν21 b1 640 (29) 640.8 (21) ν33 b2 573 (3) 576.8 (3)
Reference 24 22 This work 23
aWeak (intensity < 1 km mol−1) or inactive IR modes are unlisted.
bHarmonic vibrational wavenumbers calculated with CCSD(T*)-F12a/VDZ-F12
including anharmonic contributions calculated with B2PLYP-D/VTZ.
cIRMPD: Infrared multiphoton dissociation; IRPD: infrared photodissociation. dThe ? mark indicates that the assignment is tentative.
eRelative IR intensities normalized to the 194 km mol−1value predicted for the most
intense line for C6H7+at 1452 cm−1with the B2PLYP-D/VTZ method.
C6D6/p-H2 (1/1000) experiments is shown in TableIII. The
features pointing downward and indicated with wavenumbers 773.0, 865.6, 908.2, 953.0, 1076.1, 1272.8, 1306.6, 1430.7, and 2792.8 cm−1in Fig.4(a)belong to group B. A complete list of lines in group B observed in the C6D6/p-H2 (1/1000)
experiments is shown in TableIV.
Cordonnier et al. reported that, in p-H2 plasma, the
electron impact ionization followed by the ion-neutral reac-tion, reaction (2), produces pure p-H3+, but the subsequent
reactions between p-H3+ and p-H2 scrambles protons; the
hydrogen exchange reaction produces o-H3+ and acts as
the gateway for nuclear spin conversion.48 We did observe
weak absorption features of Q1(0) of H2 near 4153 cm−1
induced by presence of o-H2 and the Q1(1) + S0(1) band
near 4750 cm−1, indicating that a small yield for conversion to o-H2in our experiments.
V. DISCUSSION
A. Assignment of lines in group A toc-C6H7
Lines in group A, shown as positive features in Fig.3(c), increased in intensity upon UV irradiation. Because irradia-tion of the matrix at 365 nm is expected to release electrons trapped in the matrix and to neutralize the cations, it is thus expected that lines in group A are associated with a neutral species and those in group B are associated with a cation.
FIG. 4. Partial IR absorption spectra of matrix samples in regions 700−1500 and 2650−2840 cm−1. (a) Difference spectrum of the electron-bombarded C6D6/p-H2 (1/1000) sample upon irradiation at 365 nm for 2 h, (b) stick
spectrum of C6D6H+, and (c) stick spectrum of c-C6D6H simulated based
on anharmonic vibrational wavenumbers and IR intensities calculated with the B3PW91/6-311++G(2d,2p) method. Lines indicated with * are due to residues from subtraction of intense lines of C6D6.
Lines in group A are readily assigned to absorption of cy-clohexadienyl (c-C6H7) radical according to comparison with
lines previously observed in a Xe matrix.35 (TableI) and
vi-brational wavenumbers and relative IR intensities predicted quantum-chemically, as listed in TableIII.
Seven vibrational modes with lines at 2768, 1387, 1287, 958, 908, 618/620, and 546 cm−1 were reported for c-C6H7
isolated in a Xe matrix. Except the one at 546 cm−1, these features are close to the corresponding lines at 2780.1, 1394.7, 1289.9, 959.8, 910.7, and 622.0 cm−1observed in our experi-ments. We did not observe any line near 546 cm−1; the closest line was observed at 510.2 cm−1. The additional lines at 864.1 and 970.5 cm−1 in Fig.3(c)were unobserved in the Xe ma-trix, probably because they are weaker than lines at 959.8 and 910.7 cm−1. We observed in total 16 lines that can be assigned to c-C6H7, much more than the 7 lines reported for c-C6H7in
the Xe matrix. Lines reported in the Xe matrix are associated with the more intense lines observed in this work.
Observed lines agree with anharmonic vibrational wavenumbers predicted with the B3PW91/6-311++G(2d,2p) method, as shown in Figs.3(c)and3(e)and listed in TableIII. The deviations in wavenumbers less than 0.7% except ν24
which was tentatively assigned to a feature at 3056.8 cm−1 and shows a deviation of 1.0%. Observed relative IR inten-sities also agree satisfactorily with the predicted values, ex-cept those of C−H stretching modes (ν4, ν17, and ν24) which
are 2−3 times weaker than predicted. Because of the narrow spectral widths and superb sensitivity of this method, nearly all lines with predicted intensities greater than 1 km mol−1 were identified.
The C6D6/p-H2experiments provide further support for
the assignment and lines in group A observed in the C6D6
/p-H2 experiments are assigned to c-C6D6H. In Fig. 4(a) the
upward-pointing lines in group A are at 748.2, 758.1, 797.6, 831.3, 1237.8, 1246.1, 1247.7, and 2784.1 cm−1, consistent
TABLE III. Comparison of observed and theoretical vibrational wavenumbers (in cm−1) and relative IR intensities of c-C6H7and c-C6D6H.
c-C6H7 c-C6D6H
Mode Theorya p-H2b Theorya p-H2b Isotopic ratioc
ν1(a1) 3071 (14)d 3080.4 (13)d 2292 (10)d 2297.7 (12)e 0.7459 (0.7464) ν2(a1) 3035 (4) 3030.5 (3) 2273 (8) ν3(a1) 3049 (9) 3050.0 (10) 2241 (5) 2232.3 (13) 0.7319 (0.7349) ν4(a1) 2763 (52) 2780.1 (23) 2764 (68) 2784.1 (68) 1.0014 (1.0002) ν5(a1) 1564 (0) 1529 (0) ν6(a1) 1424 (5) 1425.6 (5) 1239 (3) 1237.8 (6) 0.8683 (0.8700) ν7(a1) 1405 (8) 1394.7 (9) 1239 (18) 1246.1 (19) 0.8935 (0.8823) ν8(a1) 1177 (0) 897 (0) ν9(a1) 981 (0) 953 (0) ν10(a1) 959 (6) 959.8 (5) 798 (15) 797.6 (8) 0.8310 (0.8324) ν11(a1) 867 (4) 864.1 (3) 754 (13) 748.2 (16) 0.8659 (0.8706) ν12(a1) 558 (0) 535 (0) ν13(a2) 1151 (0) 842 (1) ν14(a2) 959 (0) 757 (0) ν15(a2) 721 (0) 555 (0) ν16(a2) 375 (0) 331 (0) ν17(b1) 2755 (20) 2757.4 (10) 2041 (35) 2041.4 (8) 0.7403 (0.7407) ν18(b1) 977 (1) 970.5 (1) 836 (3) 840.0?(1) 0.8655 (0.8566) ν19(b1) 909 (9) 910.7 (7) 834 (8) 831.3 (18) 0.9128 (0.9169) ν20(b1) 965 (0) 767 (10) 758.1 (13) ν21(b1) 625 (100) 622.0 (100) 463 (100) 461.6 (100?) 0.7421 (0.7398) ν22(b1) 518 (12) 510.2 (11) 437 (38) ν23(b1) 173 (0) 149 (0) ν24(b2) 3030 (51) 3056.8? (14) 2269 (50) 2267.2 (20) 0.7417 (0.7490) ν25(b2) 3041 (2) 2250 (3) 2246.9 (7) ν26(b2) 1510 (1) 1431 (0) ν27(b2) 1387 (1) 1389.7 (1) 1116 (3) ν28(b2) 1340 (0) 1322 (0) ν29(b2) 1284 (8) 1289.9 (4) 1250 (13) 1247.7 (10) 0.9673 (0.9735) ν30(b2) 1152 (0) 826 (0) ν31(b2) 1095 (0) 996 (0) ν32(b2) 777 (0) 625 (0) ν33(b2) 587 (1) 563 (3)
aAnharmonic vibrational wavenumbers of c-C
6H7and c-C6D6H were calculated with the B3PW91/6-311++G(2d,2p) method. bThe ? mark indicates that the assignment is tentative.
cDefined as the ratio of wavenumber of the isotopic species to that of c-C
6H7; theoretical values are listed in parentheses for comparison. dRelative intensities listed in parentheses were normalized to the most intense band (ν
21) of c-C6H7and c-C6D6H which were calculated to be 89.3 and 40.2 km mol−1, respectively,
with B3PW91/6-311++G(2d,2p).
eThe ? mark indicates that the intensity could not be determined accurately due to poor signal-to-noise ratio near the wavelength limit of detection. Observed intensities are normalized
to the predicted IR intensity of the second most intense line at 2784.1 cm−1.
with those shown in Fig.4(c)for theoretically predicted stick spectrum of c-C6D6H at 754 (ν11), 767 (ν20), 798 (ν10), 834
(ν19),1239 (ν7), 1239 (ν6), 1250 (ν29), and 2764 (ν4) cm−1
according to anharmonic vibrational wavenumbers and IR intensities calculated with the B3PW91/6-311++G(2d,2p) method. The feature due to the CH-stretching mode was ob-served at 2784.1 cm−1, indicating that the isotopic variant of mono-hydrogenated benzene that we produced in this experi-ment is C6D6H.
Table III compares wavenumbers and relative IR in-tensities of all observed lines in group A in the C6H6
/p-H2 and C6D6/p-H2experiments with anharmonic vibrational
wavenumbers and IR intensities of c-C6H7 and c-C6D6H
predicted with the B3PW91/6-311++G(2d,2p) method. The agreements in wavenumbers and relative IR intensities are satisfactory. The experimental isotopic ratios agree satisfac-torily with those predicted with theory, with deviations less
than 0.009. Hence, we are confident with the assignment of features in group A to c-C6H7 and c-C6D6H in experiments
of C6H6/p-H2and C6D6/p-H2, respectively.
As shown in Table I, most vibrational modes reported previously from experiments on laser-induced fluorescence34
do not match with modes observed in IR, even though they correspond well to predicted values of vibrational modes which are IR inactive. It is unclear if these bands observed in laser-induced fluorescence were misassigned or the Franck-Condon active modes are different from the IR active modes. Further investigations are desired.
B. Assignment of lines in group B to C6H7+
The observation of the decay of these features in group B and the increase of those of in group A (c-C6H7) implies
154304-6 Bahou, Wu, and Lee J. Chem. Phys. 136, 154304 (2012)
TABLE IV. Comparison of observed and theoretical vibrational wavenumbers (in cm−1) and relative IR intensities of C6H7+and C6D6H+.
C6H7+ C6D6H+
Mode Theorya p-H2c Theorya p-H2c Isotopic ratiod
ν1(a1) 3094 (1)b 2318 (1)b ν2(a1) 3095 (3) 2285 (0) ν3(a1) 3087 (0) 2275 (1) 2278.0? (2)b ν4(a1) 2823 (38) 2813.1 (22)b 2802 (32) 2792.8 (7) 0.9928 (0.9927) ν5(a1) 1610 (41) 1603.4 (27) 1570 (55) 1567.1 (35) 0.9774 (0.9753) ν6(a1) 1455 (15) 1445.2 (8) 1278 (8) 1272.8 (10) 0.8807 (0.8778) ν7(a1) 1225 (63) 1225.5 (55) 1075 (62) 1076.1 (77) 0.8781 (0.8779) ν8(a1) 1196 (15) 1187.6 (21) 869 (3) 863.1 (3) 0.7268 (0.7267) ν9(a1) 1005 (0) 913 (1) 908.2? (1) ν10(a1) 983 (0) 953 (1) 953.0 (1) ν11(a1) 899 (8) 893.7 (10) 775 (4) 773.0 (6) 0.8649 (0.8621) ν12(a1) 588 (0) 564 (0) ν13(a2) 1106 (0) 996 (0) ν14(a2) 994 (0) 751 (2) ν15(a2) 801 (0) 615 (0) ν16(a2) 321 (0) 279 (0) ν17(b1) 2808 (16) 2798.5 (11) 2093 (18) 2093.6 (7) 0.7481 (0.7454) ν18(b1) 1040 (1) 1047.5? (1) 846 (1) 852.6 (<1) 0.8139 (0.8131) ν19(b1) 1027 (0) 884 (1) 887.3? (1) ν20(b1) 823 (9) 819.3 (9) 679 (2) 677.2 (4) 0.8266 (0.8247) ν21(b1) 648 (30) 640.8 (21) 497 (18) 492.0 (10) 0.7678 (0.7659) ν22(b1) 394 (1) 342 (1) ν23(b1) 219 (8) 183 (6) ν24(b2) 3111 (4) 2305 (1) 2295.6? (6) ν25(b2) 3074 (2) 2283 (0) ν26(b2) 1583 (0) 1475 (5) 1476.3 (6) ν27(b2) 1457 (100) 1451.9 (100) 1429 (100) 1430.7 (100) 0.9854 (0.9805) ν28(b2) 1401 (0) 1310 (15) 1306.6 (12) ν29(b2) 1339 (8) 1328.1? (7) 1100 (3) 1095.1 (12) 0.8246 (0.8211) ν30(b2) 1193 (8) 1184.8 (10) 869 (2) 865.6 (3) 0.7306 (0.7287) ν31(b2) 1128 (0) 1075.5? (4) 840 (3) ν32(b2) 971 (10) 987.6 (6) 832 (2) 845.3 (1) 0.8559 (0.8572) ν33(b2) 582 (3) 576.8 (3) 557 (3) 552.2 (4) 0.9574 (0.9582)
aAnharmonic vibrational wavenumbers of C
6H7+and C6D6H+were calculated with the B3PW91/6-311++G(2d,2p) method. bRelative intensities listed in parentheses were normalized to the most intense band (ν
27) of C6H7+and C6D6H+which were calculated to be 182.6 and 163.6 km mol−1, respectively,
with B3PW91/6-311++G(2d,2p).
cThe ? mark indicates that the assignment is tentative.
dDefined as the ratio of wavenumber of the isotopic species to that of C
6H7+; theoretical values are listed in parentheses for comparison.
maintained in the dark for an extended period or irradiated at 365 nm to release the trapped electrons; hence the carrier of these lines is most likely the corresponding cation, C6H7+.
Observed downward lines at 819.3, 893.7, 987.6, 1047.5, 1184.8, 1187.6, and 1225.5 cm−1, shown in Fig. 3(c), are consistent with corresponding anharmonic wavenumbers re-ported by Botschwina and Oswald24at 820, 888, 960, 1045,
1185, 1188, and 1239 cm−1, shown in Fig.3(d); the weak line observed at 1075.5 cm−1deviates from the predicted value of 1122 cm−1 by 4.1%. These observed line positions are also close to those reported for C6H6-Ar at 831, 903, 964, and
1058 cm−1,23with deviations 4−13 cm−1(<1.2%).
As listed in Table II, the wavenumbers of all observed features in group B are consistent with the anharmonic vibra-tional wavenumbers predicted based on harmonic vibravibra-tional wavenumbers calculated with the CCSD(T*)-F12a/VDZ-F12 method and anharmonic contributions and IR intensities cal-culated with the B2PLYP-D/VTZ method.24 They are also
compared in TableIIwith those of C6H7+recorded with the
IRMPD method22 and those of C6H7+-Ar recorded with the
IRPD method.23 Observed wavenumbers and relative IR in-tensities of C6H7+agree with those calculated theoretically,
with deviations in wavenumbers less than 1.1% except ν32
which shows a deviation of 2.9%. A stripped spectrum of C6H7+, derived on subtraction of the absorption spectrum
of C6H6and c-C6H7 from the spectrum recorded after
elec-tron impact such that most lines due to C6H6 and c-C6H7
are removed, is shown in Fig. 2(d) and compared with the IRPD spectrum of C6H7+-Ar in Fig.2(b)and the stick
spec-trum showing quantum-chemically predicted anharmonic vi-brational wavenumbers in Fig. 2(c).24 Compared with the
IRPD spectrum of C6H7+-Ar, our IR spectrum of C6H7+ in
solid p-H2 shows much narrower lines, with most lines
hav-ing widths < 0.8 cm−1, so that closely spaced lines such as those at 1184.8 and 1187.6 cm−1 are resolved, as shown in the inset of Fig. 2; these close-lying lines are also predicted
by theory at 1185 and 1188 cm−1,24as listed in TableII. The
much broader spectral coverage of the Fourier-transform IR spectrometer also enables us to observe lines at 576.8 and 640.8 cm−1that were unreported in the IRPD experiments;23
the latter line carries significant intensity. Furthermore, our spectrum presents true IR intensities that are in satisfactory agreement with theoretical predictions; lines in the C−H stretching region are much weaker than those observed with the Ar-tagging IRPD method.
Experiments on C6D6/p-H2 provide further support for
the assignments. In order to understand the deuterium iso-topic shifts, we performed calculations on both C6H7+ and
C6D6H+ with the B3PW91/6-311++G(2d,2p) method to
predict IR intensities and anharmonic vibrational wavenum-bers, as listed in Table IV. As shown in Fig.4, experimen-tal lines pointing downward (trace a) are compared with the stick spectrum of C6D6H+ predicted with the
B3PW91/6-311++G(2d,2p) method (trace b). Observed lines at 773.0, 865.6, 908.2, 953.0, 1076.1, 1272.8, 1306.6, 1430.7, and 2792.8 cm−1 are consistent with anharmonic vibrational wavenumbers predicted at 775, 869, 913, 953, 1075, 1278, 1310, 1249, and 2802 cm−1 for C6D6H+. The weak feature
observed at 2792.8 cm−1due to the CH-stretching mode sup-ports that the isotopic variant of protonated benzene that we produced in this experiment is C6D6H+.
Lists of observed wavenumbers and relative IR intensities of all lines in group B from experiments of C6H6/p-H2 and
C6D6/p-H2are compared in TableIVwith anharmonic
vibra-tional wavenumbers and IR intensities of C6H7+and C6D6H+
predicted with the B3PW91 method, respectively. The experi-mental isotopic ratios agree satisfactorily with those predicted with theory, with deviations less than 0.005. Observed relative IR intensities also agree satisfactorily with theoretical predic-tions. Hence, we are confident with the assignment of features in group B to C6H7+and C6D6H+in experiments of C6H6
/p-H2and C6D6/p-H2, respectively.
The deuterium isotopic experiments clearly support the mechanism that, upon electron bombardment of H2, H3+was
produced and a proton was transferred from H3+to C6H6(or
C6D6) to form C6H7+ (or C6D6H+); c-C6H7 (or c-C6D6H)
radicals are also produced from neutralization of C6H7+ (or
C6D6H+) or reaction of H with C6H6(or C6D6). As evidenced
by the fact that c-C6H7 and C6H7+ are the only major
prod-ucts observed in this experiment, we think that this is a much “cleaner” method to produce protonated aromatic hydrocar-bons and its neutral species.
Although the Ar-tagging IRPD method seems to pro-vide better spectra than IRMPD, the observed features are still broad. Moreover, a critical limitation of the Ar-tagging method is its difficulty in tagging a larger protonated PAH be-cause of its large internal energy; so far the largest protonated PAH detected with this method is protonated naphthalene.49 According to an astrochemical model, PAH molecules con-taining 20–80 carbon atoms are photochemically more stable in interstellar clouds.50 The similarity between the IRMPD
spectrum of protonated coronene (C24H12) and the UIE
spec-trum indicate that protonated coronene and higher PAH might contribute to the UIE bands.51 IR spectra of protonated
coronene and higher PAH with improved resolution and
spec-tral coverage are thus desirable. Our preliminary results in-dicate that this technique is also applicable to protonated coronene.
VI. CONCLUSION
Electron bombardment was applied during deposition of a mixture of C6H6 and an excess of p-H2 at 3.2 K to
gener-ate c-C6H7 and C6H7+ in the p-H2 matrix. Lines of c-C6H7
increased in intensity upon irradiation of the matrix at 365 nm, whereas those of C6H7+decreased in intensity. Observed
vibrational wavenumbers and relative IR intensities of these lines agree well with those of c-C6H7 and C6H7+ predicted
by theory. Our spectrum of c-C6H7provides twice more lines
than previous report for c-C6H7 in a Xe matrix. Our
spec-trum of C6H7+extended the spectral limit from 800 cm−1to
550 cm−1and presents true IR intensities that are in satisfac-tory agreement with theoretical predictions; line intensities in the C−H stretching region of C6H7+ observed with the
Ar-tagging IRPD method region are much larger than theoretical predictions. The IR lines of C6H7+exhibit narrow widths so
that closely spaced lines are resolved.
Our results clearly indicate that c-C6H7 and C6H7+ are
the major products and their IR spectra with much improved resolution, signal-to-noise ratio, and spectral coverage were recorded. This “clean” method provides a direct IR character-ization of the protonated aromatic and its neutral species and will be applicable to investigate the high-resolution IR spectra of larger protonated PAH and its neutral counterpart or other organic compounds52that are postulated as carriers of uniden-tified infrared emission bands in the interstellar media.
ACKNOWLEDGMENTS
National Science Council in Taiwan supported this work under the Contract No. NSC100-2745-M-009-001-ASP. Y.J.W. thanks support from Beamline 14A at National Syn-chrotron Radiation Research Center (NSRRC) in Taiwan. The National Center for High-Performance Computing provided computer time.
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