Excited state dipole moments and lifetimes of 2-cyanoindole from rotationally resolved electronic Stark spectroscopy.

The permanent dipole moments of 2-cyanoindole (cyanoindole = CNI) in its ground and lowest excited singlet states have been determined from rotationally resolved electronic Stark spectroscopy under jet-cooled conditions. From the orientation of the transition dipole moment and the geometry changes upon electronic excitation the lowest excited singlet state could be shown to be of Lb-symmetry. The general statement, that the La-state has the larger permanent dipole moment of the two lowest excited singlet states, will be challenged in this contribution. On the basis of the different electronic nature of the first excited singlet state the behavior of 2-, 3-, 4- and 5-CNI is discussed. The excited state lifetime of isolated 2-CNI in the gas phase has been determined to be 9.4 ns. This value is compared to the excited state lifetime in ethyl acetate solution of 2.6 ns, which was quantified with a Strickler-Berg analysis. Using water as solvent shortens the 2-CNI lifetime to <40 ps. The reason for this drastic shortening is discussed in detail. Additionally, the rotationally resolved electronic spectrum of 2-CNI(1-d1) has been measured and analyzed.

Determination of excited state dipole moments in solution via thermochromic methods.

The method basically combines the existing ideas of excited state dipole moment determination via thermochromic fluorescence spectroscopy with the determination of the solvent cavity volume via concentration dependent density measurements of the solution densities at different weight fractions. Additionally, the determination of the cavity volume in dependence of the solvent temperature is included here, which provides a better accuracy of the excited state dipole moment determination. With this step two major sources of errors are eliminated: the use of the very imprecise Onsager radius and the assumption, that the cavity size is temperature independent.•Thermochromic absorption and fluorescence spectroscopy.•Cavity volume determination by density measurements.•Temperature dependent cavity volume determination.

Additional data for evaluation of the excited state dipole moments of anisole.

We present the temperature dependent density, fluorescence emission and absorption spectroscopic data, that are needed for an evaluation of the excited state dipole moment of anisole in ethyl acetate via the methods of thermochromic shifts. Furthermore, the rotationally resolved electronic Stark spectrum of anisole in the molecular beam is presented. Finally, the Cartesian coordinates of the CC2/cc-pVTZ optimized structures of anisole are given in bohr units. For details about the evaluation of the dipole moments from the given data, see the connected research article: Lindic et al. (2018) [1].

Rotationally resolved electronic spectroscopy of 3-cyanoindole and the 3-cyanoindole-water complex.

The rotationally resolved electronic spectra of the origin bands of 3-cyanoindole, 3-cyanoindole(d1), and the 3-cyanoindole-(H2O)1 cluster have been measured and analyzed using evolutionary algorithms. For the monomer, permanent dipole moments of 5.90 D for the ground state, and of 5.35 D for the lowest excited singlet state have been obtained from electronic Stark spectroscopy. The orientation of the transition dipole moment is that of an 1Lb state for the monomer. The water moiety in the water cluster could be determined to be trans-linearly bound to the NH group of 3-cyanoindole, with an NHO hydrogen bond length of 201.9 pm in the electronic ground state. Like the 3-cyanoindole monomer, the 3-cyanoindole-water cluster also shows an 1Lb-like excited singlet state. The excited state lifetime of isolate 3-cyanoindole in the gas phase has been determined to be 9.8 ns, and that of 3-cyanoindole(d1) has been found to be 14.8 ns, while that of the 1 : 1 water cluster is considerably shorter (3.6 ns). The excited state lifetime of 3-cyanoindole(d1) in D2O solution has been found to be smaller than 20 ps.

Towards the Detection of Explosive Taggants: Microwave and Millimetre-Wave Gas-Phase Spectroscopies of 3-Nitrotoluene.

The monitoring of gas-phase mononitrotoluenes is crucial for defence, civil security and environmental interests because they are used as taggant for TNT detection and in the manufacturing of industrial compounds such as dyestuffs. In this study, we have succeeded to measure and analyse at high-resolution a room temperature rotationally resolved millimetre-wave spectrum of meta-nitrotoluene (3-NT). Experimental and theoretical difficulties have been overcome, in particular, those related to the low vapour pressure of 3-NT and to the presence of a CH3 internal rotation in an almost free rotation regime (V3 =6.7659(24) cm-1 ). Rotational spectra have been recorded in the microwave and millimetre-wave ranges using a supersonic jet Fourier Transform microwave spectrometer (Trot <10 K) and a millimetre-wave frequency multiplication chain (T=293 K), respectively. Spectral analysis of pure rotation lines in the vibrational ground state and in the first torsional excited state supported by quantum chemistry calculations permits the rotational energy of the molecule, the hyperfine structure due to the 14 N nucleus, and the internal rotation of the methyl group to be characterised. A line list is provided for future in situ detection.

Excited-State Dipole Moments and Transition Dipole Orientations of Rotamers of 1,2-, 1,3-, and 1,4-Dimethoxybenzene.

Rotationally resolved electronic Stark spectra of rotamers of 1,2-, 1,3-, and 1,4-dimethoxybenzene have been recorded and analyzed using evolutionary strategies. The experimentally determined dipole moments as well as the transition dipole moments are compared to the results of ab initio calculations. For the electronic ground states of the experimentally observed dimethoxybenzenes, the permanent dipole moments can be obtained from vectorial addition of the monomethoxybenzene dipole moment. However, this is not the case for the electronically excited states. This behavior can be traced back to a state mixing of the lowest electronically excited singlet states for the asymmetric rotamers. For the symmetric rotamers however, this is not valid. We discuss several possible reasons for the non-additivity of the dipole moments in the excited states of the symmetric rotamers.

Rotationally resolved electronic spectroscopy of the rotamers of 1,3-dimethoxybenzene.

Conformational assignments in molecular beam experiments are often based on relative energies, although there are many other relevant parameters, such as conformer-dependent oscillator strengths, Franck-Condon factors, quantum yields and vibronic couplings. In the present contribution, we investigate the conformational landscape of 1,3-dimethoxybenzene using a combination of rotationally resolved electronic spectroscopy and high level ab initio calculations. The electronic origin of one of the three possible planar rotamers (rotamer (0,180) with both substituents pointing at each other) has not been found. Based on the calculated potential energy surface of 1,3-dimethoxybenzene in the electronic ground and lowest excited state, we show that this can be explained by a distorted non-planar geometry of rotamer (0,180) in the S1 state.

High resolution study of the ν2 and ν5 rovibrational fundamental bands of thionyl chloride: Interplay of an evolutionary algorithm and a line-by-line analysis.

The ν2 and ν5 fundamental bands of thionyl chloride (SOCl2) were measured in the 420 cm-1-550 cm-1 region using the FT-far-IR spectrometer exploiting synchrotron radiation on the AILES beamline at SOLEIL. A straightforward line-by-line analysis is complicated by the high congestion of the spectrum due to both the high density of SOCl2 rovibrational bands and the presence of the ν2 fundamental band of sulfur dioxide produced by hydrolysis of SOCl2 with residual water. To overcome this difficulty, our assignment procedure for the main isotopologues 32S16O35Cl2 and 32S16O35Cl37Cl alternates between a direct fit of the spectrum, via a global optimization technique, and a traditional line-by-line analysis. The global optimization, based on an evolutionary algorithm, produces rotational constants and band centers that serve as useful starting values for the subsequent spectroscopic analysis. This work helped to identify the pure rotational submillimeter spectrum of 32S16O35Cl2 in the v2=1 and v5=1 vibrational states of Martin-Drumel et al. [J. Chem. Phys. 144, 084305 (2016)]. As a by-product, the rotational transitions of the v4=1 far-IR inactive state were identified in the submillimeter spectrum. A global fit gathering all the microwave, submillimeter, and far-IR data of thionyl chloride has been performed, showing that no major perturbation of rovibrational energy levels occurs for the main isotopologue of the molecule.

Communication: Molecular gears.

The (1)H nuclear magnetic resonance spectrum of hexamethylbenzene orientationally ordered in the nematic liquid crystal ZLI-1132 is analysed using covariance matrix adaptation evolution strategy. The spectrum contains over 350 000 lines with many overlapping transitions, from which four independent direct dipolar couplings are obtained. The rotations of the six methyl groups appear to be correlated due to mutual steric hindrance. Adjacent methyl groups show counter-rotating or geared motion. Hexamethylbenzene thus behaves as a molecular hexagonal gear.

On the Additivity of Molecular Fragment Dipole Moments of 5-Substituted Indole Derivatives.

The estimate of the magnitude and the orientation of molecular electric dipole moments from the vector sum of bond or fragment dipole moments is a widely used approach in chemistry. However, the limitations of this intuitive model have rarely been tested experimentally, particularly for electronically excited states. Herein, we find rules for a number of indole derivatives by using rotationally resolved electronic Stark spectroscopy and ab initio calculations. Based on a natural-bond-orbital analysis, we discuss whether the vector additivity rule can be applied in a given electronic state. From a comparison of the experimental data with ab initio calculations, we deduced that the additivity model does not apply when the flow of electron density from the substituent is opposed to that inside the chromophore.

Determination of ground and excited state dipole moments via electronic Stark spectroscopy: 5-methoxyindole.

The dipole moments of the ground and lowest electronically excited singlet state of 5-methoxyindole have been determined by means of optical Stark spectroscopy in a molecular beam. The resulting spectra arise from a superposition of different field configurations, one with the static electric field almost parallel to the polarization of the exciting laser radiation, the other nearly perpendicular. Each field configuration leads to different intensities in the rovibronic spectrum. With an automated evolutionary algorithm approach, the spectra can be fit and the ratio of both field configurations can be determined. A simultaneous fit of two spectra with both field configurations improved the precision of the dipole moment determination by a factor of two. We find a reduction of the absolute dipole moment from 1.59(3) D to 1.14(6) D upon electronic excitation to the lowest electronically excited singlet state. At the same time, the dipole moment orientation rotates by 54(∘) showing the importance of the determination of the dipole moment components. The dipole moment in the electronic ground state can approximately be obtained from a vector addition of the indole and the methoxy group dipole moments. However, in the electronically excited state, vector addition completely fails to describe the observed dipole moment. Several reasons for this behavior are discussed.

Communication: molecular dynamics and (1)H NMR of n-hexane in liquid crystals.

The NMR spectrum of n-hexane orientationally ordered in the nematic liquid crystal ZLI-1132 is analysed using covariance matrix adaptation evolution strategy (CMA-ES). The spectrum contains over 150 000 transitions, with many sharp features appearing above a broad, underlying background signal that results from the plethora of overlapping transitions from the n-hexane as well as from the liquid crystal. The CMA-ES requires initial search ranges for NMR spectral parameters, notably the direct dipolar couplings. Several sets of such ranges were utilized, including three from MD simulations and others from the modified chord model that is specifically designed to predict hydrocarbon-chain dipolar couplings. In the end, only inaccurate dipolar couplings from an earlier study utilizing proton-proton double quantum 2D-NMR techniques on partially deuterated n-hexane provided the necessary estimates. The precise set of dipolar couplings obtained can now be used to investigate conformational averaging of n-hexane in a nematic environment.

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals.

The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvents are studied over a wide temperature range and analysed using covariance matrix adaptation evolutionary strategy. Since alkanes possess small electrostatic moments, their anisotropic intermolecular interactions are dominated by short-range size-and-shape effects. As we assumed for n-butane, the anisotropic energy parameters of each n-pentane conformer are taken to be proportional to those of ethane and propane, independent of temperature. The observed temperature dependence of the n-pentane dipolar couplings allows a model-free separation between conformer degrees of order and conformer probabilities, which cannot be achieved at a single temperature. In this way for n-pentane 13 anisotropic energy parameters (two for trans trans, tt, five for trans gauche, tg, and three for each of gauche+ gauche+, pp, and gauche+ gauche-, pm), the isotropic trans-gauche energy difference Etg and its temperature coefficient Etg(') are obtained. The value obtained for the extra energy associated with the proximity of the two methyl groups in the gauche+ gauche- conformers (the pentane effect) is sensitive to minute details of other assumptions and is thus fixed in the calculations. Conformer populations are affected by the environment. In particular, anisotropic interactions increase the trans probability in the ordered phase.

NMR of short-chain hydrocarbons in nematic and smectic a liquid crystals.

The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.

EASY-GOING deconvolution: Automated MQMAS NMR spectrum analysis based on a model with analytical crystallite excitation efficiencies.

The EASY-GOING deconvolution (EGdeconv) program is extended to enable fast and automated fitting of multiple quantum magic angle spinning (MQMAS) spectra guided by evolutionary algorithms. We implemented an analytical crystallite excitation model for spectrum simulation. Currently these efficiencies are limited to two-pulse and z-filtered 3QMAS spectra of spin 3/2 and 5/2 nuclei, whereas for higher spin-quantum numbers ideal excitation is assumed. The analytical expressions are explained in full to avoid ambiguity and facilitate others to use them. The EGdeconv program can fit interaction parameter distributions. It currently includes a Gaussian distribution for the chemical shift and an (extended) Czjzek distribution for the quadrupolar interaction. We provide three case studies to illustrate EGdeconv's capabilities for fitting MQMAS spectra. The EGdeconv program is available as is on our website http://egdeconv.science.ru.nl for 64-bit Linux operating systems.

Efficient analysis of highly complex nuclear magnetic resonance spectra of flexible solutes in ordered liquids by using molecular dynamics.

The NMR spectra of n-pentane as solute in the liquid crystal 5CB are measured at several temperatures in the nematic phase. Atomistic molecular dynamics simulations of this system are carried out to predict the dipolar couplings of the orientationally ordered pentane, and the spectra predicted from these simulations are compared with the NMR experimental ones. The simulation predictions provide an excellent starting point for analysis of the experimental NMR spectra using the covariance matrix adaptation evolutionary strategy. This shows both the power of atomistic simulations for aiding spectral analysis and the success of atomistic molecular dynamics in modeling these anisotropic systems.

Nuclear magnetic resonance study of alkane conformational statistics.

NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl-4(')-cyanobiphenyl (5CB) and Merck ZLI 1132 (1132) are investigated over a wide temperature range. The ratios of dipolar couplings of ethane to propane are constant over the entire temperature range. Assuming that this constancy applies to the butane conformers facilitates the separation of probability from order parameter. This separation allows the investigation of conformational distribution without the need of invoking any model for the anisotropic intermolecular potential. The results give an order matrix that is consistent with that predicted from model potentials that describe the orientational potential in terms of short-range size and shape effects. The isotropic intermolecular potential contribution to the trans-gauche energy difference E(tg) is found to be temperature dependent with the values and variation in agreement with that found when the same results are analyzed using the chord model for anisotropic interactions [A. C. J. Weber and E. E. Burnell, Chem. Phys. Lett. 506, 196 (2011)]. The fit obtained for 9 spectra in 5CB (63 dipolar couplings) has an RMS difference between experimental and calculated dipolar couplings of 2.7 Hz, while that for the 16 spectra in 1132 (112 couplings) is 6.2 Hz; this excellent fit with nine adjustable parameters suggests that the assumption of equal temperature dependencies of the order parameters for ethane, propane, and each conformer of butane is correct. Also the fit parameters (E(tg) and the methyl angle increase) obtained for 1132 and 5CB agree. The results indicate that the chord model, which was designed to treat hydrocarbon chains, is indeed the model of choice for these chains. The temperature variation of E(tg) provides a challenge for theoreticians. Finally, even better fits to the experimental dipolar couplings are obtained when the energy in the Boltzmann factor is used for scaling ethane to butane results. However, in this case the values obtained for E(tg) differ between 1132 and 5CB.

EASY-GOING deconvolution: combining accurate simulation and evolutionary algorithms for fast deconvolution of solid-state quadrupolar NMR spectra.

A fast and accurate fit program is presented for deconvolution of one-dimensional solid-state quadrupolar NMR spectra of powdered materials. Computational costs of the synthesis of theoretical spectra are reduced by the use of libraries containing simulated time/frequency domain data. These libraries are calculated once and with the use of second-party simulation software readily available in the NMR community, to ensure a maximum flexibility and accuracy with respect to experimental conditions. EASY-GOING deconvolution (EGdeconv) is equipped with evolutionary algorithms that provide robust many-parameter fitting and offers efficient parallellised computing. The program supports quantification of relative chemical site abundances and (dis)order in the solid-state by incorporation of (extended) Czjzek and order parameter models. To illustrate EGdeconv's current capabilities, we provide three case studies. Given the program's simple concept it allows a straightforward extension to include other NMR interactions. The program is available as is for 64-bit Linux operating systems.

How and why do transition dipole moment orientations depend on conformer structure?

A remarkable influence of the orientation of a polar side chain on the direction of the S(1) ← S(0) transition dipole moment of monosubstituted benzenes was previously reported from high-resolution electronic spectroscopy. In search for a more general understanding of this non-Condon behavior, we investigated ethylamino-substituted indole and benzene (tryptamine and 2-phenylethylamine) using ab initio theory and compared the results to rotationally resolved laser-induced fluorescence measurements. The interaction of the ethylamino side chain with the benzene chromophore can evoke a rotation and a change of ordering of the molecular orbitals involved in the excitation, leading to state mixing and large changes in the orientation of the excited-state transition dipole moment. These changes are much less pronounced in tryptamine with the indole chromophore, where a rotation of the transition dipole moment is attributed to Rydberg contributions of the nitrogen atom of the chromophore. For phenylethylamine, a strong dependence of the oscillator strengths of the lowest two singlet states from the conformation of the side chain is found, which makes the use of experimental vibronic intensities for assessment of relative conformer stabilities at least questionable.