Mass-analyzed Threshold Ionization Techniques Research Articles

Effect of a single methyl substituent on the electronic structure of cobaltocene studied by computationally assisted MATI spectroscopy.

Metallocenes represent archetypical organometallic compounds playing key roles in various fields of fundamental and applied chemistry. Many of their unique properties arise from low ionization energies (IE) which can be tuned by introducing substituents into the rings. Here we report the first mass-analyzed threshold ionization (MATI) spectrum of a methylmetallocene, (Cp')(Cp)Co (Cp' = η5-C5H4Me, Cp = η5-C5H5). The presence of a single Me group allows us to study the "pure" effect of methylation without the mutual influence of substituents. The MATI technique provides an extremely high accuracy in determining the adiabatic IE of (Cp')(Cp)Co which equals 5.2097(6) eV. The effect of a Me group on the IE of cobaltocene appears to be 36% stronger than that in bis(η6-benzene)chromium. The MATI spectrum of (Cp')(Cp)Co shows a rich vibronic structure from which vibrational frequencies of the free ion are determined. This information provides a solid basis for testing the quality of quantum chemical calculations. Various levels of the DFT and coupled cluster computations are used to describe the structural and electronic transformations accompanying the detachment of an elctron from (Cp')(Cp)Co. New aspects of the methyl substituent influence on the potential energy surfaces, as well as on the inhomogeneous changes in charge density and electrostatic potential caused by ionization, are discussed.

Vibrational spectra of 2-cyanophenol cation studied by the mass analyzed threshold ionization technique

The ionization energy of 2-cyanophenol has been measured by both photoionization efficient spectroscopy and mass analyzed threshold ionization technique. The precise ionization energy of 2-cyanophenol is determined to be 73118 ± 5 cm−1 for the first time. The vibrational spectra of the cation are measured and analyzed in detail based on DFT calculations. The most active normal modes in the D0 state are assigned to the in-plane ring deformation.

Mass-analyzed threshold ionization spectroscopy of trans-o-methylanisole

In this study, we recorded the vibronic and cation spectra of trans-o-methylanisole by using resonant two-photon ionization and mass-analyzed threshold ionization techniques. The excitation energy of the S1 ← S0 transition and the adiabatic ionization energy were measured as 36,364 ± 2 cm−1 and 64,755 ± 5 cm−1, respectively. The red shifts of these values with respect to those of anisole and toluene indicated that the interactions of CH3 and OCH3 substituents with the ring are stronger in the electronically excited S1 and cationic ground D0 states than that in the electronically ground S0 state. Most of the observed bands in the S1 and D0 states involve in-plane ring deformation and methyl internal rotation. The vibrational frequencies of modes 6b1, 6a1, and 11 were lower in the S1 state than in the D0 state. This suggested that trans-o-methylanisole is more rigid in the D0 state than in the S1 state.

Selected cis- and trans-3-fluorostyrene rotamers studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

We applied two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic, photoionization efficiency, and cation spectra of the selected rotamers of 3-fluorostyrene. The adiabatic ionization energies of cis- and trans-3-fluorostyrene were determined to be 69 960±5 and 69 856±5cm−1, respectively. Cation vibrations 10a, 15, 6b, and 12 of both rotamers have been found to have frequencies of 218, 404, 452, and 971cm−1, respectively. This finding shows that the relative orientation of the vinyl group with respect to the F atom does not affect these vibrations of the 3-fluorostyrene cation. Our one-dimensional potential energy surface calculations support that the cis–trans isomerization of 3-fluorostyrene does not occur under the present experimental conditions.

Identification of four rotamers of m-methoxystyrene by resonant two-photon ionization and mass analyzed threshold ionization spectroscopy

We report the vibronic and cation spectra of four rotamers of m-methoxystyrene, recorded by using the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The excitation energies of the S1← S0 electronic transition are found to be 32 767, 32 907, 33 222, and 33 281 cm(-1), and the corresponding adiabatic ionization energies are 65 391, 64 977, 65 114, and 64 525 cm(-1) for these isomeric species. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states involve in-plane ring deformation and substituent-sensitive bending motions. It is found that the relative orientation of the methoxyl with respect to the vinyl group does not influence the vibrational frequencies of the ring-substituent bending modes. The two dimensional potential energy surface calculations support our experimental finding that the isomerization is restricted in the S1 and D0 states.

Spectroscopic Investigation of cis-2,4-Difluorophenol Cation by Mass-analyzed Threshold Ionization Spectroscopy

We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of 2,4-difluorophenol. As supported by our theoretical calculations, only the cis form of 2,4-difluorophenol involves in the two-photon photoexcitation and pulsed field ionization processes. The band origin of the S1 ← S0 electronic transition of cis-2,4-difluorophenol appears at 35 647 ± 2 cm and the adiabatic ionization energy is determined to be 70 030 ± 5 cm, respectively. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states mainly involve in-plane ring deformation vibrations. Comparing these data of cis-2,4-difluorophenol with those of phenol, cis-2fluorophenol, and 4-fluorophenol, we found that there is an additivity rule associated with the energy shift resulting from the additional fluorine substitution.

Rotamers of <em>m</em>-Methylanisole Studied by Mass-Analyzed Threshold Ionization Spectroscopy

The S-1 <- S-0 electronic transition and threshold ionization of the cis and trans rotamers of m-methylanisole were investigated by using one-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The first electronic excitation energies (E-1) of the cis and trans rotamers were determined to be (36049 +/- 2) and (36117 +/- 2) cm(-1), while the adiabatic ionization energies (I-p) were (64859 +/- 5) and (65110 +/- 5) cm(-1), respectively. The results of ab initio and density functional theory calculations provide a satisfactory interpretation for our experimental findings concerning the difference in the transitional energies of the cis and trans rotamers and assist in assigning the vibronic and cation spectra obtained in the present study. The observed active vibrations of both rotamers in the S-1 and D-0 states primarily consist of methyl torsion, in-plane ring deformation, and substituent-sensitive bending modes. Both experimental and theoretical results show that, for both cis and trans isomers, the geometry of the cation in the D-0 state is somewhat different from that of the neutral species in the S-1 state. In addition, the strengths of both the through-space substituent-substituent and substituent-ring interactions were found to follow the order: S-0<S-1<D-0.

Rotamers of 3,4-difluorophenol studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

Abstract We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibrational spectra of 3,4-difluorophenol in the first electronically excited and the ionic ground states. The band origins of the S1 ← S0 electronic transition of the cis and trans rotamers are found to be 35,486 ± 2 and 35,704 ± 2 cm−1 and the adiabatic ionization energies are 70,016 ± 5 and 70,203 ± 5 cm−1, respectively. The distinct spectral features mainly result from the in-plane ring deformation and substituent-sensitive bending vibrations. Spectral analysis suggests that the molecular geometry and vibrational coordinates of the cation in the D0 state resemble those of the neutral species in the S1 state for both cis and trans rotamers.

Mass-analyzed threshold ionization spectroscopy of trans-1-methoxynaphthalene cation and the methoxyl substitution effect

We applied the two-color resonant two-photon mass-analyzed threshold ionization technique to record the cation spectra of trans-1-methoxynaphthalene via five intermediate levels. The adiabatic ionization energy of this molecule is determined to be 61357±5cm−1, which is smaller than that of naphthalene by 4330cm−1. This indicates that the interaction between the OCH3 substituent and the aromatic ring is stronger in the cationic D0 state than that in the neutral S0 state. Comparing these data of trans-1-methoxynaphthalene with those of naphthalene, we find that the frequency difference of each mode depends on the vibrational pattern and the OCH3 group involved in the overall molecular motion.

Vibronic and cation spectroscopy of structural isomers p- and m-diaminobenzene and the amino substitution effect

► We report the R2PI and MATI spectra of p -diaminobenzene and m -diaminobenzene. ► The first electronic transition and ionization energies are precisely measured. ► The data help us to learn the amino substitution effects on molecular properties. We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of p -diaminobenzene (PDAB) and m -diaminobenzene (MDAB). The band origins of the S 1 ← S 0 electronic transition of PDAB and MDAB were found to be 29 805 ± 2 and 33 395 ± 2 cm −1 , whereas the adiabatic ionization energies were determined to be 54 624 ± 5 and 58 321 ± 5 cm −1 , respectively. Most of the observed active vibrations of these two structural isomers in the electronically excited S 1 and cationic ground D 0 states result from in-plane ring deformation and bending vibrations. Comparing the present data with those of aniline suggests that the interaction of the amino substituent and aromatic ring in the upper (S 1 or D 0 ) state is stronger than that in the S 0 state. In addition, the frequency of observed active vibrations may depend on the nature, relative location, vibrational pattern, and the extent of the amino group participated in the normal vibration.

Two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of o-chloroanisole

Abstract We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibrationally resolved spectra of the selected isotopomers of o -chloroanisole in the electronically excited S 1 and cationic ground D 0 states. As supported by our theoretical calculations, only the trans form of o -chloroanisole involves in the two-photon photoexcitation and ionization processes. The band origins of the S 1 ← S 0 electronic transition and adiabatic ionization energies for the 35 Cl and 37 Cl isotopomers of o -chloroanisole are found to be the same within our detection limit, with the values of 35 745 ± 2 and 66 982 ± 5 cm −1 , respectively. The general spectral features of the two isotopomers are nearly identical and result from in-plane ring deformation and substituent-sensitive bending and stretching as well as the CH 3 torsional motions.

Vibronic and cation spectroscopy of 2,4-difluoroaniline

We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of 2,4-difluoroaniline. The cation spectra were recorded by ionizing via the 00, X1, 6b1, and 11 levels of the electronically excited S1 state. Most of the observed active modes of this molecule in the S1 and cationic ground D0 states are related to the in-plane ring deformation vibrations. The band origin of the S1←S0 electronic excitation was found to appear at 33294±2cm−1, whereas the adiabatic ionization energy was determined to be 63935±5cm−1. Comparing the data of 2,4-difluoroaniline with those of aniline, 2-fluoroaniline, and 4-fluoroaniline, one can learn the effects of fluorine substitution on the electronic transition and molecular vibration.

Mass-analyzed threshold ionization spectroscopy of deuterium-substituted isotopomers of o-fluoroaniline and m-fluoroaniline cations

We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of deuterium-substituted isotopomers of o-fluoroaniline (OFA) and m-fluoroaniline (MFA). The origins of the S 1 ← S 0 electronic transitions and adiabatic ionization energies of these isotopomers were precisely determined.

Cation spectroscopy of 3,4-difluoroaniline by two-color resonant two-photon mass-analyzed threshold ionization

We applied the two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrationally resolved cation spectra of 3,4-difluoroaniline (34DFA) via the 0 0, X 1 , 6b 1, and I 2 levels of the S 1 state. The adiabatic ionization energy of this molecule was determined to be 64 195 ± 5 cm −1. Most of the observed active modes of the 34DFA cation in the D 0 state are related to the in-plane ring deformation vibrations. Comparing these data with those of 3-fluoroaniline and 4-fluoroaniline, one can learn the effects of fluorine substitution on the electronic transition and molecular vibration.

Selected cis- and trans-p-methoxystyrene rotamers studied by mass-analyzed threshold ionization spectroscopy

We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to investigate the ionic properties of the selected rotamers of p-methoxystyrene. The adiabatic ionization energies of cis- and trans-p-methoxystyrene are determined to be 62487±5 and 62512±5cm−1, respectively. Most of the observed active vibrations involve in-plane ring and substituent-sensitive motions. Our experimental results show that different orientation of the methoxyl group with respect to the vinyl group only slightly influence these cation vibrations, depending on the degree of involvement of the substituents involved in the overall vibrations.

Molecular structures and vibrations of cis and trans m-cresol in the electronically excited S 1 and cationic D 0 states

The optimized molecular structures of cis and trans m-cresol in the ground S 0, electronically excited S 1, and cationic D 0 states are predicted by ab initio and density functional theory (DFT) calculations. Their vibrational spectra in the S 1 and D 0 states are recorded by two color resonant (1 + 1′) two photon ionization (2C-R2PI) and mass analyzed threshold ionization (MATI) methods. In consideration of the optimized geometries, the trans rotamer is more stable than the cis one in the S 0 state. Upon the S 1 ← S 0 excitation, the aromatic ring expansion is expected, and the interaction of the OH group with the ring is enhanced. On the D 0 ← S 1 transition, the bond length of the C1–O7 bond is further shortened, exhibiting a partial double bond character in the D 0 state. The band origins of cis and trans m-cresol are measured to be 35,982 ± 2 and 36,098 ± 2 cm −1 by the 2C-R2PI method, and their adiabatic ionization energies (IE) are determined to be 66,933 ± 5 and 67,084 ± 5 cm −1 by the MATI technique. Comparison of the IE of o-, m-, p-cresol, and phenol gives the order as: p < o < m < phenol. Analysis of the spectroscopic features of cis and trans m-cresol in the S 1 and D 0 states shows that different orientations of the OH group with respect to the CH 3 group slightly influence the vibrational frequency of the in-plane ring deformation.

Rotamers of m-cresol cation studied by mass-analyzed threshold ionization spectroscopy

We have applied two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrational spectra of the selected rotamers of m-cresol. The adiabatic ionization energies of cis and trans m-cresol are determined to be 66,933 ± 5 and 67,084 ± 5 cm −1, respectively. Frequencies of the in-plane ring vibrations 6a, 1, and 9b are measured to be 528, 720, 1167 cm −1 for the cis and 520, 698, and 1153 cm −1 for the trans m-cresol cation. This indicates that different orientation of the OH group with respect to the CH 3 group slightly influences these ring vibrations.