Vibrational Spectra and Theoretical Calculations of cis- and trans-3

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Vibrational Spectra and Theoretical Calculations of cis and trans 3Fluoro-N-methylaniline in the Neutral (S) and Cationic (D) Ground States 0

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Lijuan Zhang, Sheng Liu, Min Cheng, Yikui Du, and Qihe Zhu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11991 • Publication Date (Web): 15 Dec 2015 Downloaded from http://pubs.acs.org on December 18, 2015

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The Journal of Physical Chemistry

Vibrational Spectra and Theoretical Calculations of cis and trans 3-Fluoro-N-methylaniline in the Neutral (S0) and Cationic (D0) Ground States Lijuan Zhang1,2, Sheng Liu1, Min Cheng1*, Yikui Du1*, Qihe Zhu1 1

Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China

2

Department of Chemical Engineering, Binzhou University, Binzhou, 256600, Shandong, P. R. China

* Corresponding author. Tel.: +86 10 61943129; fax: +86 10 62563167. E-mail address: [email protected] (Y. Du); [email protected] (M. Cheng).

ACS Paragon Plus Environment

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Abstract

The mass-analyzed threshold ionization (MATI) spectra of jet-cooled cis and trans 3-fluoro-N-methylaniline (3FNMA) were recorded by ionizing via the vibrationless 00 and various vibrational levels of the S1 state. The adiabatic ionization energies (IEs) of cis and trans 3FNMA are determined to be 61,742 ± 5 and 61,602 ± 5 cm-1, respectively. In the 0– 1800 cm-1 region, most of the observed vibrations in the D0 state result from the in-plane ring deformation and substituent-sensitive modes. For the high-frequency vibration region, the infrared-ultraviolet (IR-UV) double-resonance and autoionization-detected infrared (ADIR) spectroscopies were applied to investigate the N–H and C–H stretching vibrations of bare 3FNMA in the S0 and D0 states. The C–H stretching vibrational information, which we failed to

obtain

for

the

bare

3FNMA

cation,

is

complemented

by

recording

the

infrared-photodissociation (IRPD) spectra of its Ar cluster cation. It is revealed that a red-shifted frequency and an enhanced intensity are observed for the N–H stretch, while blue-shifted frequencies and greatly decreased intensities are found for both aromatic and the methyl C–H stretches. The blue-shift of the C–H stretches is firstly explained by the balance of two factors, the hyperconjugative interaction and the rehybridization effect. Analysis on the vibrational frequencies reveals a correlation between the relative stability of two rotamers in different electronic states and the relative rigidity of aromatic ring, indicating a mechanism of the long-range interactions “through bond” between the substituents. The DFT calculations can well reproduce the vibrational spectra in both S0 and D0 states. With the experimental and theoretical data, the substitution and conformation effects on the properties of 3FNMA in the S0 and D0 states, including the molecular structures, the reactive sites of electrophilic attack, and the vibrational behaviors, were discussed in detail. Keywords 3-Fluoro-N-methylaniline; MATI; IR-UV; ADIR; IRPD; Conformation effect


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1. Introduction It is known that the molecular conformation can influence the charge distribution on the molecules and consequently leads to different molecular properties of the conformers, which plays a significant role in determining the performance of a molecular system. Therefore, the understanding of the conformation of biologically relevant molecules has gained broad interest

1-4

. Being the component units of the larger and more complex flexible compounds,

the conformers of simple aromatic molecules, such as substituted anisoles and phenols

5-8

have been extensively investigated using various experimental methods, providing abundant molecular information in the neutral ground (S0), electronic excited (S1), and cationic ground (D0) states. N-methylaniline (NMA), with the CH3 substituent on the amino group of AN, has more flexibility, and displays different reactivity and kinetic pattern in the reactions relative to AN 9. Besides, as a typical secondary aromatic amine, NMA is often used as the substrate in studying some catalytic reactions10-11. The additional substituents in NMA are expected to affect the π electron interaction and molecular geometry. Investigations on the substitution effect and conformation preference in different electronic states are essential in understanding the chemical and biochemical phenomena and reactions involving them. However, the information about the molecular properties of such substituted NMAs is very limited in literature. Here, 3-fluoro-N-methylaniline (3FNMA) is chosen as a prototype to investigate the substitution and conformation effects due to the meta-fluorine substituent on NMA. Its conformation analysis in the S0 state has been performed by microwave spectroscopy

12

,

indicating the existence of two rotamers with near-planar equilibrium configuration, which are denoted by cis and trans 3FNMA, depending on the orientation of the meta-fluorine atom with respect to the amino hydrogen atom. Recently, we have investigated the molecular properties of 3FNMA rotamers in the S1 state and electronic transition energies by using the resonance-enhanced multi-photon ionization (REMPI) spectroscopy and theoretical calculations 13. It is found that the most sTable rotamer of 3FNMA is the cis one for S0 state, but the trans one for D0 state. This phenomenon may be of great importance to steric chemistry and biologic process, since the neutral and ionic molecule of different conformation may have different steric reaction, while the conformational changes upon oxidization/ionization can regulate the cellular processes 4. Extended studies on the 3FNMA


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rotamers in the S0 and D0 states will give us a comprehensive understanding on the substitution and conformation effects to the molecular properties in different electronic states, which are essential in exploring the chemical and biochemical phenomena and reactions involving such substituted NMA derivatives 3, 9. Besides, it is known that the hydrogen bond involving the N–H bond plays an important role in determining the secondary structure of peptides and proteins. The vibrational spectrum of N–H bond, due to its sensitivity to the cluster structures, has been long realized as one of the most powerful tools to identify the hydrogen bonding. Since the spectral changes strongly depend upon the strength of the interaction within the cluster, it is necessary to know the unperturbed frequencies of N–H stretching mode of bare molecule and cation. Here, our infrared (IR) investigation of 3FNMA monomer will provide a basis for the follow-up studies of its clusters, and by comparing its IR spectral features with those of aniline and its derivatives 14-15, information about the substitution and conformation effects can be obtained. Moreover, the observation of gas phase IR spectra of C–H stretches for such aromatic hydrocarbons is not only relevant to the combustion and environmental chemistry 16, but also has an important implication for investigating the astrophysical problem of the interstellar unidentified infrared emission (UIE) bands 17. Many research groups have proposed that the neutral and ionized aromatic hydrocarbons are among the candidates for the carriers of the diffuse interstellar bands

18-19

. Although the emission spectra of hot molecules may not

necessarily be same as absorption spectra of cold molecules

20

, the IR spectroscopic

properties of such prototypical aromatic compounds under conditions similar to the interstellar environments (low-temperature and collision-free) will provide us valuable reference information for the spectroscopic detection and analysis of other more complex aromatic species. The REMPI and mass-analyzed threshold ionization (MATI) methods are powerful tools to investigate the properties of polyatomic molecules and their complexes 21-22, providing the S1 and D0 state vibronic spectra. However, for the high-frequency vibrational modes such as OH, NH, and CH stretches, it is difficult to get their spectra using the above techniques, due to the poor Franck-Condon factors and band congestion

23

. The tunable infrared laser

techniques in conjunction with the time-of-flight mass spectrometry have enabled us to observe these molecular vibrations under supersonic jet expansion conditions. To record the


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IR spectrum of neutral species, an IR method coupled with the REMPI technique called infrared-ultraviolet (IR-UV) double-resonance spectroscopy is utilized, which was first applied by Page et al.

24-25

in studying benzene monomer and its cluster. By tuning the UV

laser to the selected electronic transition, and taking advantage of the mass-selected ionization detection, the IR spectrum of each species present in the jet expansion can be obtained. This technique and some of its variants have been currently used and developed by many groups worldwide

1-2, 14, 26-29

. A technique called autoionization-detected infrared

(ADIR) spectroscopy has been developed by Fujii. et al.

30

to study the vibrations of

jet-cooled molecular cations. However, due to the limitation of the technique itself or the dynamics of the Rydberg states, some vibrations might be difficult to obtain by using the ADIR method 30. In such cases, an alternative strategy is to use the “messenger” technique 31, which takes the infrared photodissociation (IRPD) spectra of van der Waals (vdW) cluster cation as the IR spectra of the corresponding bare molecule since the rare gas has a negligible perturbation to the bare molecule

14, 23, 32-34

. The combination of these IR spectroscopic

techniques above-mentioned enables us to yield and compare the vibrations of both neutral and cationic species. In this paper, we report an experimental and theoretical study on the vibronic spectra of cis and trans 3FNMA in the S0 and D0 states. The spectra of 3FNMA+ cations in the 0–1800 cm-1 region were investigated by the MATI technique. As for the high-frequency N–H and C– H stretching vibrations, the aforementioned three IR spectroscopic methods were applied to obtain a detailed vibronic analysis for the S0 and D0 states. The neutral species were probed by using IR-UV double-resonance spectroscopy, while for those of cations, both the ADIR spectra of bare 3FNMA+ and the IRPD spectra of 3FNMA+-Ar vdW cluster were recorded to acquire the complete information of the N–H and C–H stretching vibrations. Theoretical calculations were carried out to determine the geometries of 3FNMA and its Ar cluster, and help explain the observed spectral behaviors. The substitution and conformation effects on the molecular properties of two rotamers of 3FNMA in different electronic states were discussed in detail. 2. Experimental and computational methods 2.1. Experimental methods


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The apparatus used for the spectroscopic experiments is based on the home-built REMPI/MATI spectrometer as described elsewhere

35

. The major components of

experimental system, consisting of a time-of-flight mass spectrometer (TOF-MS) and a pulsed supersonic molecular beam source. Briefly, the 3FNMA (Aldrich, 97% purity) sample is seeded in argon gas of 2.0 atm, and supersonically expanded through a pulsed nozzle (General Valve, diam 0.25 mm). After being collimated by a skimmer (diam 1 mm), the molecular beam enters the ionization chamber. For REMPI/MATI spectroscopies, two counterpropagating UV lasers interacted perpendicularly with the molecular beam at 70 mm downstream from the nozzle orifice, while for the IR spectroscopies, an additional IR laser beam together with two UV lasers were introduced into the chamber, as shown in Figure 1. In the REMPI experiment, no field was applied in the ionization region (Region I) to avoid the Stark effect resulting from the static field. In the MATI experiment, both the high-n Rydberg neutrals and the non-energy-selected prompt ions were formed simultaneously in the interaction zone of laser and molecular beam. About 320 ns after the occurrence of laser pulses, a weak pulsed field of -0.5 V/cm was switched on in Region I to reject the prompt ions. Under the influence of this discrimination field, and given sufficient time (~14 µs), the prompt ions are decelerated and eliminated from detection, while the high-n Rydberg neutrals pass through Region I at the velocity of about 550 m/s. After a time delay of 14 µs, two pulsed electric fields of 333 and 3333 V/cm were switched on synchronously in Region II and III to field-ionize the high Rydberg neutrals and accelerate the produced threshold ions. After being focused by the einzel lens, the cations formed in the REMPI or MATI process flew through a 1.0-meter-long field-free tube towards a dual-stacked micro-channel plate (MCP) detector. The ion signals, amplified by a preamplifier, were then collected and analyzed by a multi-channel scaler (MCS, Stanford Research System, SR430). In the IR-UV double resonance spectroscopy for bare 3FNMA and 3FNMA-Ar cluster in the S0 state, as shown in Figure 1, the wavelength of the UV laser is fixed at the origin band of the S1←S0 transition of the molecule (or cluster), and the ions generated by the REMPI process are monitored to reflect the population in the S0 state. A pulsed IR laser beam is introduced about 50 ns prior to the UV laser, and its wavelength is scanned. When the frequency of the IR laser is resonant with the vibrational transition of the molecule (or cluster) in the S0 state, reduction of the vibrational ground-state population occurs, and a dip of the


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REMPI ion signal is observed, corresponding to the IR spectrum of the neutral precursor in the S0 state. In the ADIR spectroscopy for bare 3FNMA in the D0 state, very high Rydberg states of 3FNMA converging on its vibrationless level of the D0 state are prepared by two-color double-resonance excitation via the origin band of its S1←S0 transition. The structure of the ion core of the Rydberg molecule can be regarded as the same as that of the bare molecular ion, since the interaction between the ion core and the high Rydberg electron is extremely weak. No delay time is provided between the two UV pulses, while the IR laser pulse is introduced about 20 ns after the UV excitation. The wavelength of IR laser is scanned, and when the IR laser is resonant with the vibrational transition of the ion core, the total energy of the Rydberg states exceeds the first ionization threshold, and vibrational autoionization occurs 36. By monitoring the produced ion signals, an IR spectrum of the ion core is obtained, which can be regarded as that of the bare cation. In the IRPD spectroscopy for 3FNMA-Ar cluster in the D0 state, the cluster cation is produced by using two-color REMPI technique via its S100 intermediate level. The first UV laser wavelength is fixed to excite the neutral cluster to the vibrationless level of S1, and the second ionization UV laser is tuned to ensure that the internal energy of the prepared cluster cation is suppressed below 200 cm-1. After a delay time of 50 ns, the IR laser is introduced. When the IR laser wavelength is resonant with the vibrational transition of the cluster cation, the vibrational excitation induces the vibrational predissociation of the cluster cation, which results in the depletion of the cluster ion intensity. Monitoring the cluster ion signal as a function of the dissociation IR laser wavelength enables us to yield the IR spectrum of the cluster cation. In the experiments above, the two UV lasers are from two frequency-doubled dye lasers (Sirah Dye Laser-CSTR) pumped by a Nd: YAG laser (Spectra-Physics, Quanta-Ray 230). The tunable IR laser in the 3 µm region, with a spectral bandwidth of about 0.7 cm-1, is generated by an optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by an injection-seeded Nd: YAG laser (Continuum Surelite EX). The output power of the IR laser is kept at about 3–4 mJ/pulse, and is mildly focused by a CaF2 lens (f=700 mm). Both the UV and the IR lasers are operated at a repetition rate of 10 Hz. The synchronization of the whole system is controlled by two pulse delay generators (Stanford


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Research System, DG535 and DG645). Typical operational pressures in the source and ionization chambers are maintained at approximately 1.0 × 10-3 and 2.0 × 10-5 Pa, respectively. 2.2. Computational methods Geometry optimization and harmonic vibrational frequency calculations of 3FNMA and its Ar cluster were performed by the Gaussian 09W program package 37. The definition and the atom numbering of cis and trans 3FNMA are given in Figure 2. To conform to the earlier use 12, the rotamers of 3FNMA are defined as “cis” and “trans” according to the orientation of the meta-fluorine atom with respect to the amino hydrogen atom. B3LYP method was used for the calculations of bare 3FNMA in the S0 and D0 states, while TD-B3LYP method was applied to the S1 state. As for the 3FNMA-Ar vdW cluster, its structures in the S0 and D0 states were calculated with the long-range corrected density functional method, ωB97X-D 38, which has been shown to be of high accuracy, especially in studying the clusters with weak intermolecular interactions. The binding energies of the 3FNMA-Ar cluster in S0 and D0 states were also estimated. All stationary points were characterized as an energy minimum by verifying that all the corresponding frequencies were real. The IE of 3FNMA was obtained as the energy difference of 3FNMA in the D0 and S0 states, including the zero point energy (ZPE) correction. The calculated vibrational frequencies quoted in this paper were scaled by a certain factor to approximately correct the combined errors stemming from the basis-set incompleteness and vibrational anharmonicity. All calculations were carried out using 6-311++G** as the basis set. The Natural bond orbital (NBO) analysis was carried out by using NBO version 5.0 program package

40

39

at the B3LYP/6-311++G** level of theory. Multiwfn

was employed for electron density analysis using the wavefunction obtained from

the B3LYP/6-311++G** calculation. 3. Results and Discussion 3.1. Calculated results of 3FNMA and its Ar cluster Table 1 lists the optimized geometric parameters of cis and trans 3FNMA in the S0, S1, and D0 states. Here, some aspects about the molecular structures are addressed. In the S0 state, the NH(CH3) group displays a pyramid-like nonplanar structure, with the


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amino H atom and CH3 group being out of ring plane, while upon S1←S0 excitation and ionization, the increased electron delocalization from the nitrogen to the ring causes a planar 3FNMA with Cs symmetry. Such molecular skeleton changes are similar to those reported for aniline 41 and NMA 42-43. Since the S1←S0 transition is mainly subject to the π*←π excitation of the ring, the increased population of anti-bonding orbitals leads to ring expansion and the ring geometry distorts further from that of a perfect hexagon, with four long and two short bonds. Upon D0←S1 transition, although only few changes take place on the ring structure, the differences of the six ring C–C bond lengths become smaller than those in the S1 state, which may account for the frequency variations between the S1 and the D0 states. As for the C1–N7 bond, its bond length in cis and trans 3FNMA is greatly shortened from 1.385 and 1.384 Å in the S0 state, both to 1.324 Å in the S1 state. Since the typical C=N bond length is 1.32 Å

44

, it implies that the C1–N7 bond exhibits a partial double bond character in the S1

state, resulting from the enhanced p-π conjugation upon excitation. In the D0 state, however, the C1–N7 bond is lengthened to 1.341 Å in both rotamers, indicating a reduced conjugation compared with that in the S1 state. This may result from the removal of one electron from the lone-pair electrons of the nitrogen atom upon ionization 45-46. As for the C3–F9 bond, its bond length is shortened by 0.017 Å in both rotamers upon S1←S0 transition, and then continues to decrease by 0.013 and 0.014 Å for cis and trans 3FNMA upon D0←S1 ionization, indicating the continuously increased interaction between the fluorine atom and the aromatic ring. In order to compare with the following IR spectra of 3FNMA in the S0 and D0 states, the bond lengths of C–H and N–H bonds are also given in Table 1. The two rotamers display similar structural changes upon ionization. The bond lengths of both aromatic and methyl C– H bonds in the D0 state show varying degrees of contraction with respect to those in the S0 state, except for the ring C4–H11 bond. The NBO analysis reveals the greatest decrease of the negative charge occurs on the C4 atom among the ring carbon atoms upon ionization, from -0.3155 (-0.3174) a.u. in S0 state to -0.1091 (-0.1091) a.u. in D0 state for cis (trans) 3FNMA. This mainly results from the strong inductive effect of the neighboring F atom. Such bond lengthening effect due to the greatly decreased electron density on the C4 atom counter-balances the shortening effect from the rehybridization of the C4–H11 bond, which changes from sp2.31 in S0 state to sp2.28 in D0 state (i.e., from 30.14% to 30.50% s-character). The final results of these two effects is the lengthening of the C4-H11 bond. To further


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understand how the additional substituents affect the chemical properties of NMA, the Fukui function ( f − = ρ ( N ) − ρ ( N − 1) = ∆ρ ) is a good choice in predicting the reactive site. The

ρ ( N ) and ρ ( N − 1) present the density function of the system with N electrons and N-1 electrons, respectively. The isosurface of f − , as displayed in Figure 3, intuitively shows the sites with more net surplus of electronic distribution density. For 3FNMA, the most positive parts of f − function are localized in N7, C4 and C6, which means the para and ortho position of NH(CH3) and F substituents are favorable reactive sites for electrophilic attack. This is also in agreement with the empirical rule that the NH(CH3) group is a much stronger ortho-para director than the F atom. As for the N–H bond, the ionization results in an increase of the N7–H14 bond length by 0.007 Å. Besides, it is noticed that the structural differences on the ring of the two rotamers become smaller in the D0 state compared with those in the S0 state, indicating that the enhanced conjugation effect of the two substituents with the ring reduces the conformation effect upon ionization. The relationship between the molecular geometries and the spectroscopic behaviors will be further discussed in the following section. As for the cis and trans 3FNMA-Ar clusters, the optimized structures in the S0 state calculated at the ωB97X-D/6-311++G** level are presented in Figure 4. The Ar atom is located above the ring plane of 3FNMA, and slightly displaced from the center of the ring towards the N atom. It is noted that, due to the nonplanarity of the amino group of 3FNMA in the S0 state, there are two inequivalent binding sites for the Ar atom in this “π-bound” geometry, above or below the ring. That is, the Ar can lie on the side opposite to the amino H atom and the CH3 group (anti conformer) or on the same side (syn conformer). Our calculations for the S0 state by using different methods uniformly predict that the anti conformer is more stable than the syn conformer by 20 cm-1 or so. However, owing to the limitation of the spectral resolution, it is difficult to give an unambiguous conclusion about which conformer is more stable just from our following experimental results. Another stable geometry called “H-bound” cluster is predicted as a local minimum by calculations. In the H-bound isomer, the Ar atom lies in the plane of the ring, interacting with the H atom of the amino group. In S0 state, the binding energies of the H-bound cis and trans 3FNMA-Ar are calculated to be -0.11 and -0.14 kcal/mol at the ωB97X-D/6-311++G** level, much smaller than -0.98 and -1.04 kcal/mol, the binding energies of the π-bound cis and trans


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rotamers, respectively. The large difference of the binding energy between the H- and π-bound isomers suggests negligible population of the less stable H-bound isomer in S0 state. In S1 and D0 states, no great geometrical changes occur for the four clusters, except that the Ar atom gets closer to the ring plane in the π-bound isomer, and the Ar atom gets closer to the H atom in the H-bound isomer. In D0 state, the binding energies of π-bound cis and trans 3FNMA+-Ar cluster are calculated to be -1.28 and -1.24 kcal/mol, while those of H-bound isomers are -1.04 and -0.98 kcal/mol, respectively. The difference of their binding energies in the D0 state becomes much smaller than that in the S0 state, which may arise from the extra contribution of the electrostatic and induction interactions existing in the H-bound cations. 3.2. MATI spectra of 3FNMA The REMPI spectra of jet-cooled 3FNMA rotamers have been well studied in a previous report

13

. As shown in Figure 5(a), the band origins of the S1←S0 electronic transition are

determined to be 33,816 and 34,023 cm-1 for cis and trans 3FNMA, respectively. Figure 6 displays the MATI spectra of cis 3FNMA recorded via the vibrationless 0 00 level, the CH3 torsion τ 01 (42 cm-1), in-plane aryl C-N bending 1510 (198 cm-1), in-plane ring deformation

6a10 (455 cm-1), NHCH3 inversion I02 (564 cm-1), and ring breathing 110 (709 cm-1) vibrational levels in the S1 state. Figure 7 displays the MATI spectra of trans 3FNMA obtained via the vibrationless 0 00 level, the CH3 torsion τ 01 (68 cm-1), out-of-plane aryl C-N bending 10b01 (124 cm-1), 6a10 (450 cm-1), I 02 (538 cm-1), and 110 (699 cm-1) vibrational levels in the S1 state. Investigations of the MATI spectra through these S1 state vibrations also provide an explicit confirmation of the spectral assignments in previous REMPI spectra

13

.

The frequencies of observed MATI bands, along with the calculated values and their possible assignments, are summarized in Table 2 and 3. We tentatively assigned the vibrational modes mainly on the basis of comparison with the available data of halogen-substituted aniline 47-48 and NMA 42-43, as well as the predicted values from the DFT calculations. As shown in Figure 6 and 7, compared with the vibrational intermediates, when the vibrationless S1 000 level is used as the intermediate state, more active vibrations are observed in the MATI spectra. Analysis on the 0+ band yields the field-corrected adiabatic IEs of cis and trans 3FNMA to be 61,742 ± 5 and 61,602 ± 5 cm-1, which are in good agreement with those obtained by the photo-ionization efficiency (PIE) curves

13

, but with higher accuracy.

The B3LYP/6-311++G** calculations predict the IEs to be 60,659 and 60,518 cm-1 for cis


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and trans 3FNMA, respectively, corresponding to a deviation of only 1.8%. Both the calculations and experiments reveal that the cis rotamer has a higher IE. Most of the intense bands in the MATI spectra are corresponding to the in-plane ring deformation vibrations, as well as some active modes in the low-frequency region, involving the NHCH3 and CH3 group. When the CH3 torsion vibrational level is chosen as the intermediate state, only one peak is observed in the spectra, much less than those via other intermediate levels, which may arise from the weak coupling interaction between the CH3 group and the ring. Regardless of the overall strength of the MATI signal, it is found that most of the spectra are dominated by the same vibrational pattern as that of the intermediate S1 state, or the combination bands containing the mode of the intermediate S1 state. Such spectral behavior is in line with the propensity rule found in several aromatic molecules 47-49, suggesting that the geometry of the 3FNMA+ cation resembles that of the neutral species in the S1 state. On the other hand, as shown in Table 2 and 3, some typical vibrations involving the ring deformation and the NHCH3 group, including 6a1, 11, 121, and I2 modes, all display higher frequencies in the D0 state with respect to those in the S1 state. This fact indicates that the chemical bonds associated with these vibrations are stiffer upon D0←S1 transition, which is consistent with the calculated molecular geometries upon ionization. Comparing the spectral characters between cis and trans 3FNMA, it is found that the two rotamers display similar frequencies and activities for most vibrations in the D0 state. However, when the S1 000 , 6a10 , and 110 states are used as the intermediate levels, the MATI spectra of the cis rotamer shows more active vibrations related to the ring deformation than those of the trans one, reflecting the conformation effect due to the orientation of the CH3 group. As for the NHCH3 inversion mode, I2, the frequencies are found to be 580 and 564 cm-1 for cis and trans 3FNMA+, respectively. The red-shift of 16 cm-1 for the trans rotamer with respect to the cis one may arise from the similar reason as that in the S1 state 13. That is, due to the electron delocalization in the conjugation system, some kind of long-range interaction “through bond” seems to occur between the CH3 group and the fluorine atom in 3FNMA. As shown in Table 1, The calculated geometry reveals a smaller angle of ∠C2-C3-F9 in the trans rotamer than in the cis rotamer, and negligible difference for the ∠ C6-C5-H12 in the two rotamers. The natural population analysis (NPA) of N-methylaniline (NMA) and trans 3FNMA can give us information about the conformation effect on the


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aromatic ring due to the orientation of the CH3 group. Analysis on NMA reveals that the NPA charges on the ring C3 and C5 atoms are -0.1799 and -0.1913 a.u. in S0 state, while in D0 state, they become -0.1500 and -0.1826 a.u., respectively. The obvious different charges on the two carbon atoms, especially in D0 state, indicate that the orientation of the CH3 group indeed has a considerable influence on the distribution of the electron density on the ring. Consequently, when the H atom of NMA in the meta-position is substituted by the F atom, such kind of long-range interaction “through bond”

50

will lead to the different spectral

behaviors of cis and trans 3FNMA. Besides, it has been obtained in our previous studies

13

that the energy difference

between the two rotamers (∆Ecis-trans) in S0, S1, and D0 states are -95±25, -302±25, and 45±25 cm-1, respectively. As shown in Figure 8, some frequency differences between cis and trans rotamers (∆νcis-trans) have been given for comparison. These vibrations mainly include the typical vibrations involving the ring deformation, such as 16a1, 6a1, 11, and 121 modes. The average ∆νcis-trans of these four vibrations in S0, S1, and D0 states are 2.5, 9.5, and -4 cm-1, respectively, just in reverse order with respect to ∆Ecis-trans. Such an interesting correlation between ∆Ecis-trans and ∆νcis-trans indicates that the relative stability of the two rotamers in three electronic states may be associated with their different rigidity of the ring. This can be explained by the aforementioned “long-range interaction through bond” mechanism. The different interaction between the CH3 group and the fluorine atom in S0, S1, and D0 states, brings about different population of electron density of the rotamers, especially those on the ring, and this will results in different rigidity of the ring. On the one hand, the higher rigidity of the ring can be reflected by the higher frequencies of the ring vibrations in the spectra. On the other hand, the higher rigidity of the ring usually means the enhanced conjugation, giving lower potential energy. Therefore, it is reasonable that a correlation may exist between ∆Ecis-trans and ∆νcis-trans in S0, S1, and D0 states, demonstrating the conformation effect for the two rotamers of 3FNMA. However, more studies must be carried out to explore this potential relationship, and to supply more convinced evidence. To study the substitution effect brought by the fluorine atom and the NHCH3 group, we also compared the vibrational features of 3-fluoroaniline (3FA)

48

and NMA

43

with those of

3FNMA. It is found that whether the vibrational frequency of 3FNMA is close to that of 3FA or NMA mainly depends upon the degree of involvement of the fluorine atom and the


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NHCH3 group in the vibration motions. For example, the frequencies of 6a1 and 121, involving the in-plane ring deformation, are observed at 464(468) and 973(975) cm-1 for cis(trans) 3FNMA+; and 447 and 971 cm-1 for NMA+; and 526 and 985 cm-1 for 3FA+, respectively. It is known that the mode 6a is sensitive to the substituent along the in-plane long axis, and that is why its frequency for 3FNMA+ is much closer to that of NMA+ than to that of 3FA+. On the other hand, the transition of ring breathing 11 mode appears at 733(726), 790, and 730 cm-1, for cis(trans) 3FNMA+, NMA+, and 3FA+, respectively, indicating that the fluorine substituent on the ring has a greater effect on such vibration patterns than the CH3 substituent on the amino group. 3.3. IR-UV double resonance spectra of 3FNMA and 3FNMA-Ar neutrals Figure 5 presents the REMPI spectra of 3FNMA and its Ar cluster around the S1←S0 origin band region. The band origins of S1←S0 transition for cis and trans 3FNMA-Ar cluster are observed at 33,760 and 33,981 cm-1, respectively. The red-shifts of 56 and 42 cm-1 from the 00 bands of the monomer indicate that the Ar atom is more closely bound in the S1 state. The IR spectra of cis and trans 3FNMA in the S0 state obtained by the IR-UV double resonance spectroscopy are shown in Figure 9, along with the simulated spectra derived from the B3LYP/6-311++G** calculations. In each spectrum, the prominent band at 3488 cm-1 is assigned to the N–H stretching vibration, while the groups of bands between 2800 and 3100 cm-1 are associated with the aromatic and methyl C–H stretches. The spectral uncertainty is about ±4 cm-1. For the N–H stretching vibration, the identical frequency in the two rotamers indicates that the conformation effect plays a negligible effect on this vibrational pattern. This is also consistent with the equal N7–H14 bond length (1.007 Å) calculated for both rotamers in the S0 state. Besides, the frequency of 3488 cm-1 in the gas phase shows a blue-shift of 35 cm-1 relative to that in the condensed phase (3453 cm-1)

51

, which may be due to the

intermolecular interaction in the later. For the C–H stretches, based on the IR spectra of a series of benzene derivatives 23, 29, 51, it is known that the absorption bands above 3000 cm-1 are uniquely assigned to aromatic ring C–H stretches, while those below 3000 cm-1 are caused by alkyl C–H stretches. Here, because of the low symmetry of 3FNMA, all its C–H stretches are predicted to be active, and there are at least seven bands appearing in the spectra. As shown in Figure 9(a) and (d), the three bands at 2999, 3050, and 3090 cm-1 for cis rotamer, and 2999, 3041, and 3070 cm-1 for


14

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trans rotamer are assigned to the aromatic C–H stretching vibrations. As for the region below 3000 cm-1, which is related to methyl C–H stretching vibrations, there are at least four bands appearing in the IR spectrum for each rotamers. Since the total number of alkyl C–H stretching modes of 3FNMA is only three, three bands of alkyl C–H stretch are expected in the IR spectrum. This inconformity suggests the presence of Fermi mixing between the C–H stretching with other combination bands. The comparison between the two rotamers indicates that the C–H stretches of trans 3FNMA display low-frequency shifts of about 10–20 cm-1 with respect to those of the cis one, which may originate from the different environment for the C–H bonds due to the orientation of the CH3 group. Similar spectral behaviors have also been observed in the REMPI

13

and MATI spectra of 3FNMA, in which several vibrations

have lower frequencies for the trans rotamer, and the frequency shifts depend upon the involvement of the fluorine atom and the NHCH3 group in the vibrational motions. Besides, by comparing the methyl C–H stretching region of the two rotamers, it is found that the bands of cis 3FNMA are broader and some extra weak peaks appear in the 2950–2999 cm-1 region. This indicates stronger coupling interactions occurs in the cis rotamer between the C–H stretching modes and other vibrations. In addition, as shown in Figure 9(c) and (f), when scaled by a factor of 0.950, the calculated frequencies and oscillator strength can well reproduce the experimental data over the wavelength range investigated. The IR-UV double resonance spectra of cis and trans 3FNMA-Ar cluster are presented in Figure 9(b) and (e), with a wavelength range of 3250–3550 cm-1. The pronounced bands at 3486 and 3487 cm-1 in the spectra of cis and trans rotamer are assigned to the N–H stretching vibration, showing very small shifts with respect to that of bare 3FNMA (3488 cm-1). This indicates that the perturbation from the Ar atom is practically negligible for the N–H stretching in the S0 state, consistent with the calculated π-bound cluster geometries above. 3.4. ADIR spectra of 3FNMA cations The experimental IR spectra of cis and trans 3FNMA cations in 3350–3530 cm-1 region measured by the ADIR spectroscopy are displayed in Figure 10(a) and (d). The intense bands at 3407 and 3409 cm-1 are assigned to the N–H stretching vibration of cis and trans 3FNMA+, respectively, red-shifted by 81 and 79 cm-1 than the corresponding frequencies in S0 state. Such frequency reduction may originate from the enhanced p-π conjugation effect between the nitrogen atom and the aromatic ring upon ionization, as revealed by the calculated


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Page 16 of 40

resonance structure with a nearly double C=N bond in the D0 state. Those red-shifts of 3FNMA are much larger than those of aniline and its meta-substituted derivatives upon ionization (20–40 cm-1)

15

. The red-shifts upon ionization for the symmetric and

antisymmetric N–H stretches of 3FA are 37 and 31 cm-1

15

, respectively, similar to those of

aniline (28 and 22 cm-1). This indicates that the effect of the meta-fluorine atom on the N–H vibrations is rather small. Therefore, the large magnitude of these red-shifts for 3FNMA more likely arises from the electron-donating CH3 substituent on the amino group, which may increase the p-π conjugation to a greater degree and consequently lead to a further reduction of the N–H bond strength in the D0 state. This is in accordance with the contraction of the C1–N7 bond and the elongation of the N7–H14 bond upon ionization. Besides, it is noted that such behavior of NH group is more similar to that of the OH group in phenol, whose stretching vibrational frequency has a red-shift of 123 cm-1 upon ionization 30, 52. As for the C–H stretching vibrations, however, we failed to obtain their bands in the ADIR spectra due to the poor signal-to-noise ratio. One reason is the greatly reduced infrared absorption intensity of these vibrations upon ionization, as the findings of several alkyl-substituted benzenes and phenols ADIR method itself

23, 33

. Another reason may be the deficiency of the

30

, in which the strong field-ionization background and its fluctuation

may hide the weak signals of C–H stretching vibrations. In such cases, another approach using the Ar atom as the messenger can be used, in which the IR spectra of the weakly-bound Ar cluster are probed to indirectly reflect the vibrational information of the bare molecule. 3.5. IRPD spectra of 3FNMA-Ar cluster cations A previous IR spectroscopic study on alkylbenzenes and its Ar clusters 23 has concluded that the Ar atom gives negligible perturbation to both aromatic and alkyl C–H stretching bands. It is reasonable to expect a similar effect in the case of 3FNMA-Ar cluster, which has a similar structure as that of alkylbenzene-Ar clusters, with the Ar atom located on the aromatic ring plane, as shown in Figure 4. Although the calculations above indicated that the optimized π-bound and H-bound structures of 3FNMA+-Ar cation have comparable binding π energies, the IRPD spectra show only the ν NH band. Similarly, previous studies on the

aniline-Ar clusters 53-54 reported that no signature of the H-bound isomer was observed in the spectra using photoionization techniques. Probably, the large geometrical changes needed for the transition from the π-bound neutral minimum to the H-bound geometry cause the


16

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Franck-Condon (FC) factors too small, and only π-bound 3FNMA+-Ar cation is produced through vertical transition. Moreover, as shown in Figure 10(b) and (e), the IRPD spectra of 3FNMA-Ar cluster cation well reproduce the position of the N–H stretching band in the ADIR spectra of bare 3FNMA within the experimental error, similar to the findings in the above IR-UV double resonance spectra of neutral 3FNMA and its Ar cluster. Therefore, the spectral behaviors of bare 3FNMA+ cations can be safely deduced from the IRPD spectra of 3FNMA+-Ar cations. As shown in Figure 10(b) and (e), the IRPD spectra of 3FNMA+-Ar cations in the C–H stretching region display dramatic changes compared with those of the neutrals. The weak bands at 2892, 2946, and 2999 cm-1 are assigned as the methyl C–H stretches for cis 3FNMA, and those at 2894, 2956, and 2997 cm-1 are assigned to the methyl C–H stretches for the trans rotamer. The four bands at 3032, 3079, 3108, and 3139 cm-1 are attributed to the aromatic C– H stretches for trans 3FNMA, while for the cis rotamer at least five bands are observed in this region, with frequencies at 3039, 3082, 3105, 3116, and 3140 cm-1, respectively. It is noticed that some weak shoulder bands also appear in the 2800–3200 cm-1 region, but they are not listed here because of the difficulty in accurately determining their frequencies in the spectra with low signal-to-noise ratio. Such spectral complexity may arise from the Fermi mixing of the C–H stretches with other combination vibrations, and the more bands observed in the spectra of cis 3FNMA, especially in the aromatic C–H stretching region, indicate the stronger coupling interactions occurring in the cis rotamer. Such coupling interactions may also account for the aforementioned complicated features observed in the MATI spectra of the cis rotamer via certain ring deformation intermediate level. Besides, the frequency differences between the cis and trans rotamers become smaller in the D0 state, indicating that the enhanced conjugation effect of the two substituents with the ring reduce the conformation effect to some extent upon ionization. Compared with the C–H stretches of neutral 3FNMA, both the methyl and aromatic C– H stretching vibrations show blue-shifts by a few tens of cm-1 upon ionization. However, similar to the findings of ethylbenzene cations

23

, the methyl C–H stretches display greater

high-frequency shifts than the aromatic C–H stretches. The calculations for the S0 and D0 states, as shown in Table 1, also predict the shortening of the C–H bonds in both ring and methyl group upon ionization (except for the ring C4–H11 bond), with more contraction for


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Page 18 of 40

the methyl C–H bonds. According to the theory proposed by Alabugin et al. 55, the X–H bond length in X–H...Y hydrogen bonded complexes is controlled by a balance of two effect: the hyperconjugative X–H bond weakening and rehybridization-promoted X–H bond strengthening. Here, this mechanism is not only applicable to the hydrogen-bond complexes, but also used to understand the intramolecular interactions that affect the molecular geometry. By using the NBO analysis, the changes in hyperconjugation and hybridization of the X–H bonds of 3FNMA upon ionization were evaluated. The second-order perturbation energy E(2) is used to estimate the relative trends in n(N)→σ*(X-H) hyperconjugative interaction. Firstly, the hyperconjugation become weaker for both cis and trans 3FNMA upon ionization. Take cis 3FNMA for example, the energy of the n(N11)→σ*(C13-H14) interaction decreases from 7.76 kcal/mol in the S0 state to 2.39 kcal/mol in the D0 state. This may be attributed to the enhanced p-π conjugation effect between the nitrogen atom and the aromatic ring upon ionization, which subsequently weakens the interaction between the nitrogen atom and the CH3 group, leading to the shorter C1–N7 bond and longer C8–N7 bond in the D0 state. Secondly, the calculated results indicate that in the D0 state, all the methyl C– H bonds show an increase in the s-character at the carbon atom with varying degrees. Take cis 3FNMA for example, hybridization of the C8–H14 bond in methyl group changes from sp3.10 in the S0 state to sp2.86 in the D0 state (i.e., from 24.38% to 25.88% s-character), while the hybridization of the C8–N7 bond changes from sp2.97 to sp3.35 (i.e., from 25.17% to 22.97% s-character) upon ionization. Such changes in s-character is in good agreement with the methyl C–H bond shortening and C8–N7 bond lengthening. Therefore, the methyl C–H bond shortening of 3FNMA upon ionization can be explained by the combination of the two factors: the weakened hyperconjugative interaction and the increased s-character and polarization of the C–H bond due to the rehybridization effect. As for the aromatic C–H stretches, it is hard to give a firm explanation to their blue-shifts upon ionization, but this spectral feature seems to be a general tendency for aromatic cations. In an earlier study on benzene and other alkylbenzene cations 23, Fujii et al. proposed that a breakdown of the separability between the σ and π electrons is related to such phenomena. For 3FNMA in our experiment, the ionization, corresponding to the removal of one electron from the nitrogen atom, brings enhanced conjugation effect between the substituents and the ring. This consequently leads to the redistribution of the electron density


18

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on the whole ring and induces the rehybridization of the ring C–H bonds. Our calculations also confirm that there is indeed an increase of s-character of carbon hybrid orbital in the aromatic C–H bonds upon ionization, resulting in the bond shortening. With regard to the IR absorption intensities, each C–H stretching vibration of 3FNMA shows a greatly decreased spectral activity upon ionization. In fact, such considerable lowering of the IR intensity for the C–H stretching vibrations in the D0 state has been found in the IR spectra of several aromatic hydrocarbons

14, 32, 56

, as well as the related calculations

17

. However, in contrast to the case of toluene and ethylbenzene cations 23, whose aromatic

C–H stretches rather than the alkyl C–H stretches display greater reduction in the intensities with respect to those of the neutrals, the intensity reduction is more significant for the methyl C–H stretches in 3FNMA. Such different spectral behaviors for 3FNMA and alkylbenzene cations are also well reproduced by the theoretical calculations, and the simulated results of 3FNMA are shown in Figure 10(c) and (f). For 3FNMA, the increased molecular symmetry upon ionization may be related to its decreased IR activity of the C–H stretches. It is known that the relationship between IR intensity and dipole moment is given as I IR ∝ ψ i Mˆ ψ f , where Mˆ is the dipole moment with the Cartesian coordinates, and ψ is the wave function with the subscript i and f representing the initial and final states. The integral gives information about the probability of a transition occurring. Changes of the molecular symmetry is closely related to the transition dipole moment, and thus influences the IR intensity observed in the spectroscopy

57-58

. For 3FNMA, the increased electron

delocalization from the nitrogen to the ring causes the amino group changing from a pyramid-like nonplanar structure in S0 state to a planar geometry in D0 state with Cs symmetry. The symmetry of the C–H stretching vibrational modes is thus enhanced, and then it leads to the dipole moment changes because some polar directions cancel each other. The final result observed is the decreased absorption intensity of the IR bands of 3FNMA in D0 state. Moreover, the position of the CH3 group in 3FNMA is not as that of the alkyl substituent in alkylbenzene which is directly attached to the ring, and this distinction may result in their different relative IR activities of aromatic and alkyl C–H stretches upon ionization. Although the DFT calculations can well reproduce the experimental IR spectra qualitatively in both frequency and intensity, as well as their changes upon ionization, some


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detailed explanations for the above spectral behaviors are still ambiguous. High level calculations for the open shell system and further experimental studies for other NMA derivatives are still needed to get a deeper understanding of the phenomena, and the related work is ongoing. 4. Conclusions We obtained the MATI and IR spectra of cis and trans 3-fluoro-N-methylaniline in the S0 and D0 states, in combination with quantum chemical calculations. The MATI spectra of cis and trans rotamers were measured via their individual different S1 intermediate levels. Well-resolved vibronic structures of the cation in the 0–1800 cm-1 region show clear preference for preserving the vibrational excitation of the intermediate state, indicating that the geometry and symmetry of the 3FNMA+ ion resembles that of the neutral species in the S1 state. For the N–H and C–H stretching vibrations of 3FNMA, their spectra in S0 state is performed by using IR-UV double resonance spectroscopy. Because of the negligible perturbation of the Ar atom to bare 3FNMA, the IRPD spectra of 3FNMA-Ar cations were measured in the C–H stretching region to supplement the information that is not obtained in the ADIR spectra of bare 3FNMA cations. Remarkable differences have been found for the IR spectra of the cations and the neutrals, with red-shifted frequency and increased intensity for the N–H stretch, and blue-shifted frequencies and dramatically decreased intensities for both aromatic and methyl C–H stretches upon ionization. The frequency variations are explained by the charge delocalization due to the enhanced conjugation effect in the D0 state. The shortening of the methyl C–H bonds upon ionization were firstly explained by the balance of two factors: the weakened hyperconjugative interaction and the increased s-character and polarization of the C–H bond due to the rehybridization effect. Similar spectral features have been found for the two rotamers, and the minor discrepancies for some bands are due to the different coupling interaction between the two substituents and the Fermi mixing with other combination bands. The calculated geometrical changes are in reasonable agreement with the observed spectral behaviors. Besides, comparison of the relevant vibrations observed in the spectra reveals a correlation between the relative stability of two rotamers in different electronic states and the relative rigidity of the ring, which may arise from the long-range interactions “through bonds” between the substituents. This possible


20

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correlation would be helpful in predicting conformational properties of more complex molecules, and needs further investigations. Author Information Corresponding Author * E-mail address: [email protected] (Y. Du); [email protected] (M. Cheng). Notes The authors declare no competing financial interest. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grants No. 20973180, 21190034, 21203207, and 21173236. This research was also supported by the Research Fund of Binzhou University, China (No. 2014Y18). References (1) Zwier, T. S. Laser Spectroscopy of Jet-Cooled Biomolecules and Their Water-Containing Clusters: Water Bridges and Molecular Conformation. J. Phys. Chem. A 2001, 105, 8827-8839. (2) Brause, R.; Fricke, H.; Gerhards, M.; Weinkauf, R.; Kleinermanns, K. Double Resonance Spectroscopy of Different Conformers of the Neurotransmitter Amphetamine and Its Clusters with Water. Chem. Phys. 2006, 327, 43-53. (3) Islam, M. M.; Bhuiyan, M. D. H.; Bredow, T.; Try, A. C. Theoretical Investigation of the Nonlinear Optical Properties of Substituted Anilines and N,N-Dimethylanilines. Comput. Theor. Chem. 2011, 967, 165-170. (4) Lu, K. P.; Finn, G.; Lee, T. H.; Nicholson, L. K. Prolyl Cis-Trans Isomerization as a Molecular Timer. Nat. Chem. Biol. 2007, 3, 619-629. (5) Fujimaki, E.; Fujii, A.; Ebata, T.; Mikami, N. Autoionization-Detected Infrared Spectroscopy of Intramolecular Hydrogen Bonds in Aromatic Cations. I. Principle and Application to Fluorophenol and Methoxyphenol. J. Chem. Phys. 1999, 110, 4238-4247. (6) Yosida, K.; Suzuki, K.; Ishiuchi, S.; Sakai, M.; Fujii, M.; Dessent, C. E. H.; Müller-Dethlefs, K. The PFI-ZEKE Photoelectron Spectrum of m-Fluorophenol and Its Aqueous Complexes: Comparing Intermolecular Vibrations in Rotational Isomers. Phys. Chem. Chem. Phys. 2002, 4, 2534-2538. (7) Shiung, K. S.; Yu, D.; Huang, H. C.; Tzeng, W. B. Rotamers of m-Fluoroanisole Studied by


21


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Two-Color Resonant Two-Photon Mass-Analyzed Threshold Ionization Spectroscopy. J. Mol. Spectrosc. 2012, 274, 43-47. (8) Zhang, L. J.; Yu, D.; Dong, C. W.; Cheng, M.; Hu, L. L.; Zhou, Z. M.; Du, Y. K.; Zhu, Q. H.; Zhang, C. H. Rotamers and Isotopomers of 3-Chloro-5-Fluoroanisole Studied by Resonant Two-Photon Ionization Spectroscopy and Theoretical Calculations. Spectrochim. Acta A 2013, 104, 235-242. (9) Emokpae, T. A.; Isanbor, C. Relative Reactivity and Kinetic Pattern of Aniline and N-Methylaniline as Nucleophiles in Aromatic Substitution (SNAr) Reactions. Int. J. Chem. Kinet. 2004, 36, 188-196. (10) Uchimaru, Y. N-H Activation vs. C-H Activation: Ruthenium-Catalysed Regioselective Hydroamination of Alkynes and Hydroarylation of an Alkene with N-Methylaniline. Chem. Commun. 1999, 1133-1134. (11) Garcia, P.; Payne, P. R.; Chong, E.; Webster, R. L.; Barron, B. J.; Behrle, A. C.; Schmidt, J. A. R.; Schafer, L. L. Easily Assembled, Modular N,O-Chelating Ligands for Ta(V) Complexation: A Comparative Study of Ligand Effects in Hydroaminoalkylation with N-Methylaniline and 4-Methoxy-N-Methylaniline. Tetrahedron 2013, 69, 5737-5743. (12) Cervellati, R.; Scappini, F. Conformational Analysis of N-Methyl-m-Fluoroaniline by Microwave Spectroscopy. J. Mol. Struct. 1980, 62, 81-83. (13) Zhang, L. J.; Liu, S.; Dong, C. W.; Cheng, M.; Du, Y. K.; Zhu, Q. H; Zhang, C. H. REMPI Spectroscopy and Theoretical Calculations of Cis and Trans 3-Fluoro-N-Methylaniline. J. Mol. Spectrosc. 2014, 296, 28-35. (14) Nakanaga, T.; Ito, F.; Miyawaki, J.; Sugawara, K.; Takeo, H. Observation of the Infrared Spectra of the NH2-Stretching Vibration Modes of Aniline-Arn (n=1,2) Clusters in a Supersonic Jet Using REMPI. Chem. Phys. Lett. 1996, 261, 414-420. (15) Honda, M.; Fujii, A.; Fujimaki, E.; Ebata, T.; Mikami, N. NH Stretching Vibrations of Jet-Cooled Aniline and Its Derivatives in the Neutral and Cationic Ground States. J. Phys. Chem. A 2003, 107, 3678-3686. (16) Visser, T.; Sarobe, M.; Jenneskens, L. W.; Wesseling, J. W. Identification of Isomeric Polycyclic Aromatic Hydrocarbons (PAH) in Pyrolysates from Ethynylated PAH by Gas Chromatography-Fourier Infrared Spectroscopy. Their Relevance for the Understanding of PAH Rearrangement and Interconversion Processes During Combustion. Fuel 1998, 77, 913-920. (17) Langhoff, S. R. Theoretical Infrared Spectra for Polycyclic Aromatic Hydrocarbon Neutrals, Cations, and Anions. J. Phys. Chem. 1996, 100, 2819-2841. (18) Schlemmer, S.; Cook, D. J.; Harrison, J. A.; Wurfel, B.; Chapman, W.; Saykally, R. J. The Unidentified Interstellar Infrared Bands - PAHs as Carriers. Science 1994, 265, 1686-1689. (19) Kwok, S.; Zhang, Y. Unidentified Infrared Emission Bands: PAHs or MAONs? Astrophys. J. 2013,


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771, 5. (20) Cook, D. J.; Schlemmer, S.; Balucani, N.; Wagner, D. R.; Harrison, J. A.; Steiner, B.; Saykally, R. J. Single Photon Infrared Emission Spectroscopy: A Study of IR Emission from UV Laser Excited PAHs Between 3 and 15 µm. J. Phys. Chem. A 1998, 102, 1465-1481. (21) Dessent, C. E. H.; Müller-Dethlefs, K. Hydrogen-bonding and van der Waals Complexes Studied by ZEKE and REMPI Spectroscopy. Chem. Rev. 2000, 100, 3999-4021. (22) Braun, J. E.; Neusser, H. J. Threshold Photoionization in Time-of-Flight Mass Spectrometry. Mass. Spectrom. Rev. 2002, 21, 16-36. (23) Fujii, A.; Fujimaki, E.; Ebata, T.; Mikami, N. Infrared Spectroscopy of CH Stretching Vibrations of Jet-Cooled Alkylbenzene Cations by Using the "Messenger" Technique. J. Chem. Phys. 2000, 112, 6275-6284. (24) Page, R. H.; Shen, Y. R.; Lee, Y. T. Local Modes of Benzene and Benzene Dimer, Studied by Infrared-Ultraviolet Double-Resonance in a Supersonic Beam. J. Chem. Phys. 1988, 88, 4621-4636. (25) Page, R. H.; Shen, Y. R.; Lee, Y. T. Infrared-Ultraviolet Double Resonance Studies of Benzene Molecules in a Supersonic Beam. J. Chem. Phys. 1988, 88, 5362-5376. (26) Krauss, O.; Brutschy, B. Intramolecular Charge Transfer in Jet-Cooled Methyl 4-N,NDimethylaminobenzoate·(Water)n Clusters Studied by Infrared Depletion Spectroscopy Observed in the Dispersed Fluorescence. Chem. Phys. Lett. 2001, 350, 427-433. (27) Simons, J. P.; Jockusch, R. A.; Carcabal, P.; Hung, I.; Kroemer, R. T.; Macleod, N. A.; Snoek, L. C. Sugars in the Gas Phase. Spectroscopy, Conformation, Hydration, Co-operativity and Selectivity. Int. Rev. Phys. Chem. 2005, 24, 489-531. (28) Biswal, H. S.; Wategaonkar, S. O-H···O versus O-H···S Hydrogen Bonding. 3. IR-UV Double Resonance Study of Hydrogen Bonded Complexes of p-Cresol with Diethyl Ether and Its Sulfur Analog. J. Phys. Chem. A 2010, 114, 5947-5957. (29) Cockett, M. C. R.; Miyazaki, M.; Tanabe, K.; Fujii, M. Isomer Selective IR-UV Depletion Spectroscopy of 4-Fluorotoluene-NH3: Evidence for π-Proton-Acceptor and Linear Hydrogen-Bonded Complexes. Phys. Chem. Chem. Phys. 2011, 13, 15633-15638. (30) Fujii, A.; Iwasaki, A.; Ebata, T.; Mikami, N. Autoionization-Detected Infrared Spectroscopy of Molecular Ions. J. Phys. Chem. A 1997, 101, 5963-5965. (31) Okumura, M.; Yeh, L. I.; Myers, J. D.; Lee, Y. T. Infrared Spectra of the Cluster Ions H7O3+·H2 and H9O4+·H2. J. Chem. Phys. 1986, 85, 2328-2329. (32) Dopfer, O.; Olkhov, R. V.; Maier, J. P. Infrared Photodissociation Spectra of the C-H Stretch Vibrations of C6H6+-Ar, C6H6+-N2, and C6H6+-(CH4)1-4. J. Chem. Phys. 1999, 111, 10754-10757. (33) Fujii, A.; Fujimaki, E.; Ebata, T.; Mikami, N. Autoionization-Detected Infrared Spectroscopy of


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Page 24 of 40

Jet-Cooled Aromatic Cations in the Gas Phase: CH Stretching Vibrations of Isolated p-Ethylphenol Cations. Chem. Phys. Lett. 1999, 303, 289-294. (34) Mizuse, K.; Fujii, A. Infrared Photodissociation Spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): Messenger-Dependent Balance Between H3O+ and H5O2+ Core Isomers. Phys. Chem. Chem. Phys. 2011, 13, 7129-7135. (35) Dong, C. W.; Zhang, L. J.; Liu, S.; Hu, L. L.; Cheng, M.; Du, Y. K.; Zhu, Q. H.; Zhang, C. H. REMPI and MATI Spectroscopic Study of Selected Cis and Trans 3-Chlorostyrene Rotamers. J. Mol. Spectrosc. 2013, 292, 35-46. (36) Lefebvre-Brion, H.; Field, R. W. Perturbations in the Spectra of Diatomic Molecules. Academic Press: Orlando, FL, 1986. (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. GAUSSIAN 09 (Revision A.02), Gaussian Inc., Wallingford CT, 2009. (38) Walker, N. R.; Walters, R. S.; Tsai, M. K.; Jordan, K. D.; Duncan, M. A. Infrared Photodissociation Spectroscopy of Mg+(H2O)Arn complexes: Isomers in progressive microsolvation. J. Phys. Chem. A 2005, 109, 7057-7067. (39) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0.; Theoretical Chemistry Institute, University of Wisconsin: Madison, 2001. (40) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580-592. (41) Sinclair, W. E.; Pratt, D. W. Structure and Vibrational Dynamics of Aniline and Aniline-Ar from High Resolution Electronic Spectroscopy in the Gas Phase. J. Chem. Phys. 1996, 105, 7942-7956. (42) Yu, H. P.; Joslin, E.; Phillips, D. Large-Amplitude Vibrations and Their Effects on Lowering Onset of Intramolecular Vibrational Redistribution in Jet-Cooled N-Methylaniline. J. Chem. Soc., Faraday Trans. 1993, 89, 2345-2354. (43) Wu, R. H.; Lin, J. L.; Lin, J.; Yang, S. C.; Tzeng, W. B. Mass Analyzed Threshold Ionization Spectroscopy of N-Methylaniline, N-Ethylaniline, and N,N-Dimethylaniline Cations: Influence of N-Alkyl Substitution on the Ionization Energy and Molecular Vibration. J. Chem. Phys. 2003, 118, 4929-4937. (44) Pople, J. A.; Gordon, M. Molecular Orbital Theory of Electronic Structure of Organic Compounds .1. Substituent Effects and Dipole Moments. J. Am. Chem. Soc. 1967, 89, 4253-4261. (45) Lin, J. L.; Tzeng, W. B. Mass Analyzed Threshold Ionization of Deuterium Substituted Isotopomers of Aniline and p-Fluoroaniline: Isotope Effect and Site-Specific Electronic Transition. J. Chem. Phys. 2001, 115, 743-751. (46) Lin, J.; Lin, J. L.; Tzeng, W. B. Mass Analyzed Threshold Ionization Spectroscopy of Deuterium Substituted N-Methylaniline and N-Ethylaniline Cations: Isotope Effect on Transition Energy and Large


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Amplitude Vibrations. Chem. Phys. 2003, 295, 97-107. (47) Song, X. B.; Yang, M.; Davidson, E. R.; Reilly, J. P. Zero Kinetic Energy Photoelectron Spectra of Jet-Cooled Aniline. J. Chem. Phys. 1993, 99, 3224-3233. (48) Lin, J. L.; Lin, K. C.; Tzeng, W. B. Species-Selected Mass-Analyzed Threshold Ionization Spectra of m-Fluoroaniline Cation. Appl. Spectrosc. 2001, 55, 120-124. (49) He, Y. G.; Wu, C. Y.; Kong, W. Observation of Rotamers of m-Aminobenzoic Acid: Zero Kinetic Energy Photoelectron and Hole-Burning Resonantly Enhanced Multiphoton Ionization Spectroscopy. J. Chem. Phys. 2004, 121, 8321-8328. (50) Antal, Z.; Warburton, P. L.; Mezey, P. G. Electron Density Shape Analysis of a Family of Through-Space and Through-Bond Interactions. Phys. Chem. Chem. Phys. 2014, 16, 918-932. (51) Varsányi, G. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives. Wiley: New York, 1974. (52) Fujii, A.; Sawamura, T.; Tanabe, S.; Ebata, T.; Mikami, N. Infrared Dissociation Spectroscopy of the OH Stretching Vibration of Phenol-Rare Gas van der Waals Cluster Ions. Chem. Phys. Lett. 1994, 225, 104-107. (53) Nakanaga, T.; Ito, F.; Miyawaki, J.; Sugawara, K.; Takeo, H. Observation of the Infrared Spectra of the NH2-Stretching Vibration Modes of Aniline-Arn (n=1,2) Clusters in a Supersonic Jet Using REMPI. Chem. Phys. Lett. 1996, 261, 414-420. (54) Solca, N.; Dopfer, O. Interaction Between Aromatic Amine Cations and Nonpolar Solvents: Infrared Spectra of Isomeric Aniline+-Arn (n=1,2) Complexes. Eur. Phys. J. D 2002, 20, 469-480. (55) Alabugin, I. V.; Manoharan, M.; Peabody, S.; Weinhold, F. Electronic Basis of Improper Hydrogen Bonding: A Subtle Balance of Hyperconjugation and Rehybridization. J. Am. Chem. Soc. 2003, 125, 5973-5987. (56) Piest, H.; von Helden, G.; Meijer, G. Infrared Spectroscopy of Jet-Cooled Cationic Polyaromatic Hydrocarbons: Naphthalene. Astrophys. J. 1999, 520, L75-L78. (57) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy. Dover Publications, INC: New York, U.S., 1989. (58) Philip, R. B.; Per, J. Molecular Symmetry and Spectroscopy, 2nd Ed. NRC Research Press: Ottawa, Canada, 1998.


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Table 1 The calculated geometric parameters of cis and trans 3FNMA in the S0, S1, and D0 states. a cis

trans

S0

S1

D0

S0

S1

D0

C1-C2

1.4087

1.4353

1.4288

1.4049

1.4306

1.4229

C2-C3

1.3798

1.3638

1.3678

1.3862

1.3665

1.3692

C3-C4

1.3881

1.4043

1.4098

1.3829

1.4066

1.4130

C4-C5

1.3924

1.4033

1.4045

1.3980

1.4012

1.4013

C5-C6

1.3932

1.3751

1.3767

1.3864

1.3728

1.3768

C6-C1

1.4068

1.4372

1.4337

1.4106

1.4402

1.4381

C1-N7

1.3848

1.3238

1.3410

1.3842

1.3238

1.3408

C8-N7

1.4507

1.4552

1.4639

1.4505

1.4532

1.4625

C3-F9

1.3592

1.3419

1.3287

1.3594

1.3419

1.3275

C2-H10

1.0839

1.0834

1.0837

1.0811

1.0796

1.0811

C4-H11

1.0817

1.0823

1.0834

1.0816

1.0822

1.0834

C5-H12

1.0843

1.0821

1.0827

1.0842

1.0830

1.0825

C6-H13

1.0816

1.0799

1.0813

1.0850

1.0837

1.0842

N7-H14

1.0072

1.0457

1.0146

1.0072

1.0468

1.0143

C8-H15

1.0904

1.1022

1.0874

1.0904

1.1007

1.0873

C8-H16

1.0990

1.0929

1.0922

1.0991

1.0930

1.0925

C8-H17

1.0938

1.0929

1.0922

1.0940

1.0930

1.0925

∠C6-C5-H12

119.1

119.9

120.0

119.2

120.0

120.1

∠C2-C3-F9

118.0

119.7

119.6

117.5

119.2

119.4

Bond length (Å)

Bond angle (°)

a

Calculations for the S0, S1, and D0 states are performed using B3LYP, TD-B3LYP, and UB3LYP

methods with the 6-311++G** basis set, respectively.


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Table 2 Observed bands (in cm-1) in the MATI spectra of cis 3FNMA and their possible assignments. a 0

0

94 156 210 368 408

Intermediate level in the S1 state τ(CH3) 151 6a1 I2 76 98 158 207 224 362

1

1

109 214 258 363 412 467 491

464 492 531

527 555

617

580 613

733

Calc. b

733

616 733

764 800 819 857

822 850 888

894

919 973

921 977 989

1082 1114 1168

1090 1142 1177 1195 1226 1314

1533

1528

1542

1553 1596 1707

1607

Assignment and approx. description c τ(CH3) τ(CH3) 10b1, γ(aryl C-N) 151, β(aryl C-N) γ(aryl C-N)+τ(CH3) β(N-CH3)+ β(C-F) 16a1, γ(C-C-C) 6a1, β(C-C-C) 6b1, β(C-C-C) τ(CH3)6a1 τ(CH3)6b1 I2, NHCH3 inversion 41, γ(C-C-C) 11, breathing 10b26a1 τ(CH3)11 16a2, γ(C-C-C) γ(aryl C-H) 10b111 18a1, β(C-H) 121, β(C-C-C) 16a1I2 18b1, β(C-H) 9b1, β(C-H) 9a1, β(C-H) 6a111 6b111 I211 8b1, ν(C-C) I2121 8a1, ν(C-C) 11 121

a

Experimental values are shifts from 61,742 cm-1. Calculated at the B3LYP/6-311++G** level, scaled by 0.990. c ν denotes stretching; β denotes in-plane bending; γ denotes out-of-plane bending. b

27 ACS Paragon Plus Environment


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Table 3 Observed bands (in cm-1) in the MATI spectra of trans 3FNMA and their possible assignments. a 00 103 124

Intermediate level in the S1 state τ(CH3) 10b1 6a1 I2

11

Calc. b

127 146

168 211 225 369

170 210 226

120 198 248 397

413 425

425 444

494 573 722 846

468 498

496

461 496

726

725

564 721

723 893 913 960

975 1092

977 1096 1142

1167

1175 1190 1219 1288 1337 1446

1533 1543 1596

1535 1541 1601

Assignment and approx. description c τ(CH3) τ(CH3) τ(CH3) 10b1, γ(aryl C-N) 151, β(aryl C-N) γ(aryl C-N)+τ(CH3) β(N-CH3)+ β(C-F) 152, β(aryl C-N) 16a1, γ(C-C-C) γ(aryl C-N)+τ(CH3) 6a1, β(C-C-C) 6b1, β(C-C-C) I2, NHCH3 inversion 11, breathing 16a2, γ(C-C-C) 10b111 10b2I2 6a16b1 121, β(C-C-C) 18b1, β(C-H) 10b1121 9a1, β(C-H) 6a111 6b111 I211 10b19a1 12 I2121 8b1, ν(C-C) 8a1, ν(C-C)

a

Experimental values are shifts from 61,602 cm-1. b Calculated at the B3LYP/6-311++G** level, scaled by 0.990. c ν denotes stretching; β denotes in-plane bending; γ denotes out-of-plane bending.


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Figure captions Figure 1.

Schematic diagram of TOF-MS for IR/UV spectroscopy. (For REMPI and MATI spectroscopies, only the UV lasers are introduced.)

Figure 2.

Molecular structures and labeling of cis and trans 3FNMA.

Figure 3.

Isosurface of f

-

for cis and trans 3FNMA (iso-value=0.009) obtained using

Multiwfn package. Green and blue regions correspond to the positive and negative regions, respectively. Figure 4.

Optimized structures of cis and trans 3FNMA-Ar cluster, calculated at the ωB97X-D /6-311++G** level.

Figure 5.

REMPI spectra of (a) 3FNMA and (b) 3FNMA-Ar cluster around their origin bands of the S1←S0 transition.

Figure 6.

MATI spectra of cis 3FNMA via different S1 intermediate levels. The energy in the x-axis is relative to the ionization threshold at 61,742 cm-1.

Figure 7.

MATI spectra of trans 3FNMA via different S1 intermediate levels. The energy in the x-axis is relative to the ionization threshold at 61,602 cm-1.

Figure 8.

Plot of the frequency difference between cis and trans 3FNMA (∆νcis-trans) for some typical vibrational modes in S0, S1, and D0 states. The inset in the upper right shows the relative stability of the two rotamers in three electronic states (see ref. 13). The values of ∆ν(S1) and ∆ν(D0) are obtained from the REMPI and MATI spectra, respectively; and those of ∆ν(S0) are calculated at the B3LYP/6-311++G** level . The last “average” means the average values of ∆νcis-trans for the four vibrations.

Figure 9.

(a) and (d) IR-UV double resonance spectrum of cis and trans 3FNMA in S0 state. (b) and (e) IR-UV double resonance spectrum of 3FNMA-Ar cluster in S0 state in the N–H stretching region. (c) and (f) Simulation of the IR spectrum of 3FNMA in S0 state based on the B3LYP/6-311++G**calculations. A scaling factor of 0.950 is applied to the calculated C–H and N–H stretching frequencies.

Figure 10. (a) and (d) ADIR spectrum of bare cis and trans 3FNMA in D0 state in the N–H stretching region. (b) and (e) IRPD spectrum of cis and trans 3FNMA-Ar cluster in D0 state. (c) and (f) Simulation of the IR spectrum of cis and trans 3FNMA in D0 state based on the B3LYP/6-311++G**calculations. A scaling factor of 0.956 is applied to the calculated C–H and N–H stretching frequencies. 29 ACS Paragon Plus Environment


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Figure 1 Schematic diagram of TOF-MS for IR/UV spectroscopy. (For REMPI and MATI spectroscopies, only the UV lasers are introduced.)


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cis

trans

Figure 2 Molecular structures and labeling of cis and trans 3FNMA.



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cis

Page 32 of 40

trans

Figure 3 Isosurface of f - for cis and trans 3FNMA (iso-value=0.009) obtained using Multiwfn package. Green and blue regions correspond to the positive and negative regions, respectively.


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(a) cis 3FNMA-Ar

(b) trans 3FNMA-Ar

Figure 4 Optimized structures of cis and trans 3FNMA-Ar cluster, calculated at the ωB97X-D /6-311++G** level.



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Figure 5 REMPI spectra of (a) 3FNMA and (b) 3FNMA-Ar cluster around their origin bands of the S1←S0 transition.


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Figure 6 MATI spectra of cis 3FNMA via different S1 intermediate levels. The energy in the x-axis is relative to the ionization threshold at 61,742 cm-1.



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Figure 7 MATI spectra of trans 3FNMA via different S1 intermediate levels. The energy in the x-axis is relative to the ionization threshold at 61,602 cm-1.


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Figure 8 Plot of the frequency difference between cis and trans 3FNMA (∆νcis-trans) for some typical vibrational modes in S0, S1, and D0 states. The inset in the upper right shows the relative stability of the two rotamers in three electronic states (see ref. 13). The values of ∆ν(S1) and ∆ν(D0) are obtained from the REMPI and MATI spectra, respectively; and those of ∆ν(S0) are calculated at the B3LYP/6-311++G** level . The last “average” means the average values of ∆νcis-trans for the four vibrations.



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Figure 9 (a) and (d) IR-UV double resonance spectrum of cis and trans 3FNMA in S0 state. (b) and (e) IR-UV double resonance spectrum of 3FNMA-Ar cluster in S0 state in the N–H stretching region. (c) and (f) Simulation of the IR spectrum of 3FNMA in S0 state based on the B3LYP/6-311++G**calculations. A scaling factor of 0.950 is applied to the calculated C–H and N–H stretching frequencies.


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Figure 10 (a) and (d) ADIR spectrum of bare cis and trans 3FNMA in D0 state in the N–H stretching region. (b) and (e) IRPD spectrum of cis and trans 3FNMA-Ar cluster in D0 state. (c) and (f) Simulation of the IR spectrum of cis and trans 3FNMA in D0 state based on the B3LYP/6-311++G**calculations. A scaling factor of 0.956 is applied to the calculated C–H and N–H stretching frequencies.



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Table of Contents (TOC) Graphic


Vibrational Spectra and Theoretical Calculations of cis- and trans-3

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