Condensed Phase Spectra Research Articles

Gas-Phase Characterization of Redox-Active Manganese Complexes.

Manganese complexes exhibit a rich redox chemistry, usually accompanied by structural reorganization during the redox processes often followed by ligand dissociation or association. The push-pull ligand 2,6-diguanidylpyridine (dgpy) stabilizes manganese in the oxidation states +II, +III, and + IV in the complexes [Mn(dgpy)2]n+ (n = 2-4) without change in the coordination sphere in the condensed phase [Heinze et al., Inorganic Chemistry, 2022, 61, 14616]. In the condensed phase, the manganese(IV) complex is a very strong oxidant. In the present work, we investigate the stability and redox activity of the MnIV complex and its counterion (PF6-) adducts in the gas phase, using two modified 3D Paul ion trap mass spectrometers. Six different cationic species of the type [Mnx(dgpy)2(PF6)y]n+ (x = II, III, IV, y = 0-3, n = 1-3) could be observed for the three oxidation states MnIV, MnIII, and MnII, of which one observed complex also contains a reduced dgpy ligand. MnII species showed the highest relative stability in collision induced dissociation and UV/vis photo dissociation experiments. The lowest stability is observed in the presence of one or more counterions, which correlates to a lower total charge n+. Gas phase UV/vis spectra show similar features as the condensed phase spectra only differing in relative band intensities. The strongly oxidizing MnIV complex reacts with triethylamine (NEt3) in the gas phase to give MnIII, while MnIII species show little reactivity toward NEt3.

Multidimensional Pattern Recognition in High-Resolution 2D and 3D Spectra of Gas-Phase Molecules

ConspectusWhen molecules transition from the condensed phase to the gas phase, their spectra undergo a dramatic transformation as well; each peak in a condensed-phase spectrum can yield thousands of peaks in the gas phase because the molecules are free to rotate and those rotational motions are quantized. These gas-phase spectra contain a wealth of detailed information about molecular structure and behavior, but peak densities are often so high that congestion obscures the patterns needed to assign peaks and extract molecular constants. This Account describes how coherent multidimensional techniques not only reduce peak densities and congestion in gas-phase spectra but also create multidimensional patterns that are easy to recognize and interpret. First, all peaks with the same vibrational quantum numbers form rotational patterns such as X's, double parabolas, and asterisks. These rotational patterns are composed of basic units and can provide immediate information about the molecule's structure, behavior, and rotational selection rules. Second, groups of these rotational patterns can be arranged into vibrational patterns that form arrays of rectangles or parallelograms. These vibrational patterns can be used to determine wave-mixing processes and measure vibrational constants. Coherent multidimensional spectroscopy therefore automatically separates vibrational and rotational information and then sorts peaks by vibrational and rotational quantum number. Furthermore, if the sample is composed of a mixture, then these patterns can also sort peaks by species, and higher-dimensional techniques can even provide the ability to select a species in the mixture. These techniques have successfully produced highly patterned 2D and 3D spectra for samples that otherwise generate patternless spectra such as isotopologue mixtures and vibronically perturbed molecules such as NO2.High densities of states can lead to congestion and perturbations that make it difficult to accurately assign peaks using the information that is traditionally available from 1D spectra: a peak's intensity and its frequency. Coherent 2D and 3D techniques are well-suited for dealing with and learning from perturbations because the coordinate of each peak in multidimensional space includes multiple frequency values. Accurate assignments are possible when peaks in 2D or 3D spectra that are perturbed along one frequency axis are unperturbed along an orthogonal frequency axis. Furthermore, patterns often repeat in adjacent rows or columns, so regions that are less congested can be used to resolve or identify key peaks or patterns in regions that are severely congested. Perturbations can make the spacings within multidimensional rotational and vibrational patterns slightly irregular, but these automatically generated patterns remain easy to recognize and analyze.This Account describes three high-resolution coherent multidimensional spectroscopy techniques, the types of patterns they can produce, and how information can be extracted from these patterns. This work is being conducted at Spelman College, a historically Black college for women where all of the students are undergraduates. The resulting techniques are not only highly effective for dealing with some of the most congested, perturbed, and challenging spectroscopic systems, but they are relatively easy to use, moderate in price to set up, and quick to run.

Interference between Franck-Condon and Herzberg-Teller Terms in the Condensed-Phase Molecular Spectra of Metal-Based Tetrapyrrole Derivatives.

The commonly used Franck–Condon (FC) approximation is inadequate for explaining the electronic spectra of compounds that possess vibrations with substantial Herzberg–Teller (HT) couplings. Metal-based tetrapyrrole derivatives, which are ubiquitous natural pigments, often exhibit prominent HT activity. In this paper, we compare the condensed phase spectra of zinc–tetraphenylporphyrin (ZnTPP) and zinc–phthalocyanine (ZnPc), which exhibit vastly different spectral features in spite of sharing a common tetrapyrrole backbone. The absorption and emission spectra of ZnTPP are characterized by a lack of mirror symmetry and nontrivial temperature dependence. In contrast, mirror symmetry is restored, and the nontrivial temperature-dependent features disappear in ZnPc. We attribute these differences to FC–HT interference, which is less pronounced in ZnPc because of a larger FC component in the dipole moment that leads to FC-dominated transitions. A single minimalistic FC–HT vibronic model reproduces all the experimental spectral features of these molecules. These observations suggest that FC–HT interference is highly susceptible to chemical modification.

Optical spectra in the condensed phase: Capturing anharmonic and vibronic features using dynamic and static approaches.

Simulating optical spectra in the condensed phase remains a challenge for theory due to the need to capture spectral signatures arising from anharmonicity and dynamical effects, such as vibronic progressions and asymmetry. As such, numerous simulation methods have been developed that invoke different approximations and vary in their ability to capture different physical regimes. Here, we use several models of chromophores in the condensed phase and ab initio molecular dynamics simulations to rigorously assess the applicability of methods to simulate optical absorption spectra. Specifically, we focus on the ensemble scheme, which can address anharmonic potential energy surfaces but relies on the applicability of extreme nuclear-electronic time scale separation; the Franck-Condon method, which includes dynamical effects but generally only at the harmonic level; and the recently introduced ensemble zero-temperature Franck-Condon approach, which straddles these limits. We also devote particular attention to the performance of methods derived from a cumulant expansion of the energy gap fluctuations and test the ability to approximate the requisite time correlation functions using classical dynamics with quantum correction factors. These results provide insights as to when these methods are applicable and able to capture the features of condensed phase spectra qualitatively and, in some cases, quantitatively across a range of regimes.

Linear absorption spectra of solvated thiouracils resolved at the hybrid RASPT2/MM level

On the example of 2-, 4- and 2,4-thiouracil we demonstrate the performance of the RASPT2/RASSCF protocol in reproducing the spectral positions and line shapes of linear absorption spectra that have been recorded in water and documented in this work. Through a QM/MM scheme coupled to a room-temperature Wigner sampling we simulate condensed phase spectra, permitting to compare our results against experiments. We discuss the sensitivity of the simulations to: a) the active space size by pushing the limits beyond the full-valence active spaces; b) the consideration of “dark” nπ∗-states in the state averaging; c) the flavor of RASPT2 technique; d) the basis set. The benchmarking demonstrates that full-π valence active spaces tend to red-shift the absorption band. Increasing the active space rectifies the problem and we obtain near-quantitative agreement between our experiments and calculations. We, furthermore, demonstrate that the choice of RASPT2 flavor has to be made through rigorous benchmarking.

Photodissociation of Free Metalloporphyrin Dimer Multianions.

We have used action photofragmentation spectroscopy in the visible spectral range (410 to 650 nm) to investigate the optical properties of different monomeric and dimeric M(II)-meso-tetra-(4-sulfonatophenyl)-porphyrin (with M = Pd(II), Cu(II), Zn(II)) multianions isolated in the gas phase without solvent. In particular, we report the position of the Q-bands (S0 → S1 transitions) as a function of charge state, counterions, oligomerization, and dimer structure type. The results for the monomers (charge states = 4- and 3-, sodiated and protonated) are in good agreement with TDDFT calculations and condensed-phase spectra. For both homo and heterometallic dimers, photofragmentation spectra were recorded for two charge states, 5- and 3-, corresponding to coplanar and cofacial structure types, respectively. The fragmentation patterns observed for the dimers depend significantly on charge state, with fragmentation into monomers being dominant for the 5- species, while the 3- charge state predominantly fragments by SO2 loss. The monomer → dimer Q-band spectral shifts observed in the gas phase were compared with the optical properties of porphyrin aggregates in solution.

Ethynylene-analogues of hemicurcuminoids: Synthesis and ground- and excited properties of their boron difluoride complexes

The synthesis, characterization and (TD)-DFT calculations of the electrochemical and photophysical properties of novel ethynylene-analogues of hemicurcuminoids are described. These dyes are both emissive in solution and in the solid state. While compounds that emit through an efficient charge transfer (CT) state show solvatochromic behaviour associated with low fluorescence quantum yields, those lacking of donor groups show high fluorescence quantum yields of 70–80%, in solution. The latter dyes also present the advantage to emit in the solid state in the visible region with fluorescence quantum yields up to 23%. Their condensed phase spectrum can be bathochromically shifted to the near infrared region (742 nm) by appending a strong donor group.

How to calculate linear absorption spectra with lifetime broadening using fewest switches surface hopping trajectories: a simple generalization of ground-state Kubo theory.

In this paper, we develop a surface hopping approach for calculating linear absorption spectra using ensembles of classical trajectories propagated on both the ground and excited potential energy surfaces. We demonstrate that our method allows the dipole-dipole correlation function to be determined exactly for the model problem of two shifted, uncoupled harmonic potentials with the same harmonic frequency. For systems where nonadiabatic dynamics and electronic relaxation are present, preliminary results show that our method produces spectra in better agreement with the results of exact quantum dynamics calculations than spectra obtained using the standard ground-state Kubo formalism. As such, our proposed surface hopping approach should find immediate use for modeling condensed phase spectra, especially for expensive calculations using ab initio potential energy surfaces.

Vibrational spectra of silatranes and germatranes XM(OCH2CH2)3N (X = F,Cl,H; M = Si,Ge). The problem of the theoretical prediction of condensed phase spectra

The structures of silatranes and germatranes XM(OCH2CH2)3N (X=F,Cl,H; M=Si,Ge) were optimized and their vibrational spectra were calculated at the B3LYP/aug-cc-pVDZ level of theory. Theoretical frequencies of vibrations perpendicular to the C3 axis (E type) are in good agreement with experimental values, while the axial vibrations (MX and M…N stretchings) demonstrate a significant discrepancy with experimental spectra recorded for the crystalline state. This discrepancy stems from the well-known difference in the MX and M…N bond lengths in gas and solid state. The force constant scaling procedure was used to compensate for this difference. As a result a set of scaling factors was refined for 1-Cl-germatrane (the unique atrane for which the distinction between A and E modes was experimentally established). This set was transferred to the theoretical force fields of other atranes, which provided a fair reproduction of their experimental frequencies. The analysis of the normal modes allowed us to assign the ν M…N mode to bands in the 180–270cm−1 frequency range, although large contributions of these coordinates are in two other modes in the 450–500cm−1 and 600–800cm−1 frequency ranges.The frequencies of degenerate vibrations (with vectors perpendicular to the C3 axis) do not depend substantially on the axial atom (X and M) substitution, while those of A-type in the 200–700cm−1 frequency range vary significantly.

IRMPD spectroscopic investigation of gas-phase complexes of deprotonated penicillin G with Ba 2+, Zn 2+ and Cd 2+

The bare deprotonated penicillin G, and its complexes with the Ba 2+, Zn 2+ and Cd 2+ ions, have been formed by electrospray ionization and studied by IRMPD (infrared multiple photon dissociation) spectroscopy using FT-ICR (Fourier-transform ion cyclotron resonance) mass spectrometry with the FELIX free electron laser infrared light source. The spectra in the 900–2000 cm −1 range have been interpreted by comparison with DFT-computed spectra. The spectrum of deprotonated penicillin G is in good agreement with calculations for a mixture of the two most favorable conformations, and is in good accord with the condensed-phase spectrum, except that the symmetric and antisymmetric carboxylate peaks are split much farther apart in the gas phase. The Ba(Penicillin-H) + ion is assigned as the simple complex with barium chelated by all three carbonyl oxygens, along with a cation-π interaction with the phenyl ring. The Zn(Penicillin-H) + spectrum is consistent with simple complexation in a similar tridentate conformation, probably without cation-π interaction, but the agreement with the calculated spectrum is not excellent, and rearrangement is not ruled out. The Cd(Penicillin-H) + complex probably has the simple tridentate conformation with strong cation-π interaction when sprayed from methanol solution, but rearranges when sprayed from acetonitrile to a new spectroscopically unique conformation, which was not identified with confidence. For this latter complex, the most stable calculated structure found has a deprotonated amide nitrogen and a cadmium–nitrogen covalent bond.

An ab Initio Calculation of the Valence Excitation Spectrum of H2O···Cl2: Comparison to Condensed Phase Spectra

Valence electronic excitation spectra are calculated for the H(2)O...Cl(2) dimer using state-of-the art ab initio potentials for both the ground and the valence excited states, a basis set calculation of the ground state nuclear wave function, and a wave packet analysis to simulate the dynamics on the excited state surface. The peak of the H(2)O...Cl(2) dimer spectrum is blue-shifted by 1250 cm(-1) from that of the free Cl(2) molecule. This is less than the value previously estimated from vertical excitation energies but still significantly more than the blue shift in aqueous solution and clathrate-hydrate solid. Seventy percent of the blue shift is attributed to ground state stabilization, the rest to excited state repulsion. Spin-orbit effects are found to be small for this dimer. Homogeneous broadening is found to be slightly smaller for the dimer than for the free Cl(2). The reflection principle and spectator model approximations were tested and found to be quite satisfactory. This is promising for an eventual simulation of the condensed phase spectra.

Free Carboxylate Stretching Modes

We report the first IR spectroscopic observation of carboxylate stretching modes in free space, i.e., in the complete absence of solvent or counterions. Gas-phase spectra of a series of benzoate anions have been recorded and compared to condensed-phase spectra, revealing the profound influence of the environment on the symmetric and antisymmetric carboxylate stretch modes.

Stepwise Solvatochromism of Ketyl Anions in the Gas Phase: Photodetachment Excitation Spectroscopy of Benzophenone and Acetophenone Radical Anions Microsolvated with Methanol

Electronic absorption spectra of bare and methanol-solvated radical anions of benzophenone ((C6H5)2CO) and acetophenone ((C6H5)CH3CO) were measured by monitoring the photodetachment efficiency in the gas phase. Strong absorption bands due to autodetachment after transitions to bound excited states were observed. Stepwise spectral shifts approaching the limit of the condensed phase spectra were found to occur as the cluster size increases. In the case of benzophenone radical anion, the solvation of two methanol molecules exhibits the near convergence to the limit, representing the full coordination with the solvent molecules around the carbonyl group. For the acetophenone case, the coordination number was not apparently determined because of their relatively small shifts. Relationships between hydrogen bonding and electronic structure are analyzed for the spectral shifts with the aid of calculations based on density functional theory. The calculational results show that the coordination angle of the solvent molecule is affected mostly by steric hindrance around the carbonyl group, and that there is no evidence for reorientation due to specific hydrogen bonding interaction with the singly occupied orbital, which has been formerly persisted for an interpretation of the transient absorption following pulse radiolysis in alcoholic solutions. An alternative possibility involving deformation with respect to intramolecular coordinates is discussed.

Matrix effects in the carbon 1s near edge x-ray absorption fine structure spectra of condensed alkanes

The carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectra of simple gaseous alkane molecules differ from the spectra of the same alkane molecules in the condensed phase. The origin of these large, systematic differences is poorly understood. The NEXAFS spectra of gaseous alkanes are interpreted as a progression of core-->Rydberg transitions with distinctive vibronic structure. The interpretation of the NEXAFS spectra of condensed phase alkanes is varied. Specifically, the degree of Rydberg character in the pre-edge core excited states of condensed alkanes is controversial. We determined the character of core excited states in condensed alkanes with a combination of experiment and computational study. From this, we have determined the nature of matrix effects for these species. The high-resolution carbon 1s NEXAFS spectrum of gaseous neopentane is dramatically different from its condensed phase spectrum, a striking illustration of the dramatic spectroscopic changes that occur upon condensation. High quality ab initio calculations of a cluster designed to model the solid phase environment provide definitive evidence for the reduction of Rydberg character and support the assignment of sigma*C-H) valence character in the pre-edge features in the NEXAFS spectra of condensed alkanes.

Valence electronic structure of gas-phase 3,4,9,10-perylene tetracarboxylic acid dianhydride: Experiment and theory

We present gas phase ultraviolet photoemission spectra of 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) and compare them to condensed phase spectra. Contrary to common expectations for organic solids, we observe that the two differ in many details, necessitating a reexamination of our theoretical understanding of the PTCDA electronic structure. Thus, we use density functional theory with several exchange-correlation functionals, as well as the GW method. We find that the local density approximation and the generalized gradient approximation fail to describe the experimental data because of underbound orbitals that are concentrated on the anhydride groups. We show that this is corrected by either a hybrid functional or the GW method and use the results to interpret the experimental data.

Conformational study of ketoprofen by combined DFT calculations and Raman spectroscopy

A conformational study of ketoprofen was carried out by both density functional theory (DFT) calculations and Raman spectroscopy. Nine different geometries were found to correspond to energy minimum conformations but only one of them was experimentally detected in the condensed phase spectra. Those rotations which interconvert the five most stable conformers were studied and the intramolecular interactions governing the corresponding conformational preferences were assessed. A thorough vibrational analysis was performed, leading to the assignment of both the solid and liquid spectra. Evidence for formation of intermolecular hydrogen bonds between carboxylic groups of adjacent ketoprofen molecules, leading to dimeric entities, was obtained.

A sequential molecular mechanics/quantum mechanics study of the electronic spectra of amides.

We report gas-phase electronic spectra of formamide, N-methyformamide, acetamide, and N-methylacetamide at 300 K calculated using a combination of classical molecular dynamics and time-dependent density functional theory (TDDFT). In comparison to excitation energies computed using the global minima structures, the valence npi* and pi(nb)pi* states show a significant red-shift of 0.1-0.35 eV, while smaller shifts are found for the n3s and pi(nb)3s Rydberg states. In this work, we have identified the physical origin of these shifts arising from variations of the molecular structure. We present simple relationships between key geometrical parameters and spectral shifts. Consequently, electronic spectra can be generated directly from ground-state structures, without additional quantum chemical calculations. The electronic spectrum of formamide in aqueous solution is computed using TDDFT using an explicit solvent model. This provides a quantitative determination of the condensed-phase spectrum. In general, this study shows that temperature effects can change the predicted excitation energies significantly and demonstrates how electronic spectra at elevated temperatures can be computed in a computationally efficient way.

Gas-phase electronic absorption spectroscopy of substituted bis(η6-benzene)chromium derivatives: Rydberg transitions in bis(η6-anisole)chromium and bis(η6-2,6-dimethylpyridine)chromium

Gas-phase electronic absorption spectra of chromium bisarene complexes with oxygen- and nitrogen-containing ligands, (η6-PhOMe)2Cr (1) and (η6-2,6-Me2C5H3N)2Cr (2), were first measured. Rydberg bands disappearing on going to the condensed-phase spectra were revealed. The first ionization potentials of complexes 1 and 2 (5.30 and 5.40 eV, respectively) were determined from the Rydberg frequencies. The Rydberg transitions were assigned and the corresponding Rydberg term values and quantum defects were determined. The effect of heteroatoms on the Rydberg structure parameters was revealed by comparing the spectra of complexes 1 and 2 with those of unsubstituted analogs. The appearance, in the ligand side chain, of an oxygen atom capable of being involved in conjugation with the π-electron system of the aromatic ring results in substantial broadening of the observed Rydberg bands. This can be associated with an increased ligand contribution to the HOMO of the sandwich compound. The influence of the oxygen atom on the ionization energy of the molecule is insignificant. In contrast to this, introduction of a nitrogen atom into the carbocycle leads to a noticeable increase in the ionization potential of the molecule, while the ligand contribution to the HOMO of the complex remains practically unchanged.

TPD and HREELS reinvestigation of ethylene oxide adsorption on Pt()

Abstract A reinvestigation of ethylene oxide (EtO) adsorption and desorption on Pt(1 1 1) is reported using primarily TPD, AES, XPS and HREELS. EtO does not react when adsorbed on Pt(1 1 1) at 90 K and then subsequently heated under UHV conditions. Specifically, TPD results and AES measurements taken after TPD experiments place an upper limit on the extent of EtO decomposition at 0.02 ML. This work helps to correct earlier work that showed significant decomposition on this surface. HREELS identified molecular EtO adsorption at 90 K, with only small shifts in molecular vibrational frequencies from condensed phase spectra. At a coverage of less than 0.1-ML EtO, EtO is bonded oxygen-end down to the Pt(1 1 1) surface with C 2v symmetry, i.e., with the C 2 axis of EtO normal to the surface. By showing that the close-packed Pt(1 1 1) surface is less reactive toward the desired product of ethylene epoxidation than previously thought, these new results should be helpful in discussing differences between the chemistry of Ag and Pt catalysts for this reaction.

Optimal spectrum estimation in statistical mechanics.

Better than a Fourier transform? More accurate and more flexible, principal component analysis is introduced as an alternative to Fourier transforming correlation functions to calculate condensed-phase spectra (see picture). Analytical proof and numerical tests illustrate the potential of principal mode analysis for calculating optimal vibrational spectra from quantum-mechanical/molecular-mechanical trajectories. In principal, the method is also appropriate for calculating other spectra and for analyzing Monte Carlo simulations or experimental data.