N5- in Solution: Isotopic Labeling and Further Details of Its Synthesis by Phenyl Pentazole Reduction.

The cyclopentazolate anion, N5-, has been researched extensively over the years and detected in the gas phase more than a decade ago, but was only recently measured in solution. The process whereby aryl pentazole reduction leads to the production of N5- is still not fully understood. Here, the production of N5- in solution was investigated using isotopic labeling techniques while implementing changes to the synthesis methodologies. 15N labeled phenyl pentazole produced appropriately labeled phenyl pentazole radical anions and N5- which, upon collision induced dissociation, produced the expected N3- signals. Changing to higher purity solvent and less coated Na metal allowed for a much more rapid pace, with experiments taking less time. However, the best yields were obtained with heavily coated metal and much longer reaction times. Utilization of a vacuum line and ultrapure solvents led to no products being detected, indicating the importance of a sodium passivation layer in this reaction and the possibility that sodium is too strong a reducer. These findings can lead to better production methods of N5- and also explain past failures in implementing aryl pentazole reduction techniques.

Detection of Cyclo-N5- in THF Solution.

Compelling evidence has been found for the formation and direct detection of the cyclopentazole anion (cyclo-N5- ) in solution. The anion was prepared from phenylpentazole in two steps: reduction by an alkali metal to form the phenylpentazole radical anion, followed by thermal dissociation to yield cyclo-N5- . The reaction solution was analyzed by HPLC coupled with negative mode mass spectrometry. A signal with m/z 70 was eluted about 2.1 min after injection of the sample. Its identification as N5 was supported by single and double labeling with 15 N, which yielded signals at m/z=71 and 72, respectively, with identical retention times in the HPLC column. MS/MS analysis of the m/z=70 signal revealed a dissociation product with m/z=42, which can be assigned to N3- . To our knowledge this is the first preparation of cyclo-N5- in the bulk. The compound is indefinitely stable at temperatures below -40 °C, and has a half-life of a few minutes at room temperature.

Preparation of the Cyclopentazole Anion in the Bulk: A Computational Study.

The cyclopentazole anion (cyclo-N5(-)), calculated to be a stable species, was prepared in the gas phase but attempts to synthesize it in the bulk have so far been futile. An aryl pentazole radical anion was suggested as a promising precursor in the gas phase. It is shown computationally that the radical anion (which may be prepared by reduction of the phenyl pentazole neutral) may indeed be used to form the cyclopetazolate anion in the gas phase and in liquid solution, alongside and in competition with the extrusion of N2 to produce the corresponding azide. In the gas phase, the C-N dissociation yields are very low due to much more efficient detachment of an electron. In polar solvents, ionization is suppressed and the primary yields of the two competing reactions are similar. The reaction must be carried out at low temperatures and special measures have to be taken to avoid recombination of the nascent cyclo-N5(-) with the geminate phenyl radical. A possible remedy is to use a solvent that reacts efficiently with the phenyl radical by H atom transfer.

Role of Rydberg states in the photostability of heterocyclic dimers: the case of pyrazole dimer.

A new route for the nonradiative decay of photoexcited, H-bonded, nitrogen-containing, heterocyclic dimers is offered and exemplified by a study of the pyrazole dimer. In some of these systems the N(3s) Rydberg state is the lowest excited singlet state. This state is formed by direct light absorption or by nonradiative transition from the allowed ππ* state. An isomer of this Rydberg state is formed by H atom transfer to the other component of the dimer. The newly formed H-bonded radical pair is composed of two radicals (a H-adduct of pyrazole, a heterocyclic analogue of the NH(4) radical) and the pyrazolium π-radical. It is calculated to have a shallow local minimum and is the lowest point on the PES of the H-pyrazole/pyrazolium radical pair. This species can cross back to the ground state of the original dimer through a relatively small energy gap and compete with the H-atom loss channel, known for the monomer. In both Rydberg dimers, an electron occupies a Rydberg orbital centered mostly on one of the two components of the dimer. This Rydberg Center Shift (RCS) mechanism, proposed earlier (Zilberg, S.; Kahan, A.; Haas, Y. Phys. Chem. Chem. Phys. 2012, 14, 8836), leads to deactivation of the electronically excited dimer while keeping it intact. It, thus, may explain the high photostability of the pyrazole dimer as well as other heterocyclic dimers.

The photo-dissociation of the pyrrole-ammonia complex--the role of hydrogen bonding in Rydberg states photochemistry.

The photochemistry of the pyrrole-ammonia cluster is analyzed theoretically. Whereas in neat pyrrole the dominant photochemical reaction is H-atom cleavage, recent experiments show that in pyrrole-ammonia clusters the major reaction is H-transfer to form the NH(4) radical (solvated by ammonia molecules in the case of large clusters) and the pyrrolyl radical. A mechanism involving the hydrogen-bonded Rydberg state is offered to account for these results and verified computationally. Two minima are located on the lowest excited singlet PES. Both of them are Rydberg states, one leads to the formation of NH(4) and pyrrolyl radicals, the other is connected to the πσ* state through a relatively high barrier, leading to a 3-body dissociation reaction to form a pyrrolyl radical, ammonia and an H-atom. The former is the energetically and statistically preferred one.

Frequency upshift in BO2 and CO2+ upon electronic excitation: a twin-state model rationalization.

The twin-state model, previously shown to provide a simple physical rationalization for the frequency upshift (exaltation) of the Kekulé mode in benzene upon S(0) to S(1) electronic excitation, is extended to the case of the BO(2) radical and the CO(2)(+) radical cation. In the case of BO(2)/CO(2)(+), the ground and excited states are degenerate, yet the model applies to the degenerate two-state system as well. In contrast with a pseudo-Jahn-Teller model, the twin-state one can predict which frequency is exalted and also which pair of electronic states are coupled, thus explaining the specificity of the phenomenon. The frequency exaltation is a spectroscopic manifestation of the resonance between the pair of VB structures describing twin states. In analogy with the case of benzene, it is predicted that the ν(3) asymmetric stretch fundamental will be a dominant peak in the two-photon absorption spectrum of BO(2) and CO(2)(+).

Solvent tuning of a conical intersection: direct experimental verification of a theoretical prediction.

We report an ultrafast study of a merocyanine molecule, whose fluorescence lifetime was tuned by changing the solvent's polarity. A recent theoretical prediction that the fluorescence lifetime is considerably shortened upon lowering the polarity of the solvent, due to tuning of the conical intersection properties, is fully confirmed (Xu et al. J. Phys. Chem. A 2009, 113, 9779-9791). This constitutes a direct measurement of a previously predicted tunable property of a conical intersection.

Towards experimental determination of conical intersection properties: a twin state based comparison with bound excited states.

The energy and approximate structure of certain S(0)/S(1) conical intersections (CI) are shown computationally to be deducible from those of two bound states: the first triplet (T(1)), which is iso-energetic with the CI, and the second excited singlet state (S(2)). This is demonstrated for acepentalene (I) and its perfluoro derivative (II) using the twin state concept for three states systems and based on the fact that the triplet T(1) is almost degenerate with the CI. The stable S(2) (C(3v) configuration) state exhibits unusual exaltation of Jahn-Teller active degenerate mode-ν(JT) = 2058 cm(-1) (∼500 cm(-1) higher than analogous e-mode of the symmetric (C(3v)) T(1) and the dianion I(-2) or any C-C vibration of the Jahn-Teller distorted (C(s)) ground state minimum). The acepentalene molecule, whose rigid structure and possibility to attain the relatively high symmetry C(3v) configuration, is a particularly suitable candidate for this purpose.

The photophysics of a polar molecule in a nonpolar cryogenic glass--the effects of dimerization on (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP).

The first excited state of BIDP was shown in a previous communication to exhibit an ultrafast decay in fluid nonpolar, nonprotic solutions due to the presence of a S(1)/S(0) conical intersection (CI). In frozen polar and nonpolar glasses a strong fluorescence was observed, rationalized by the hindering of the internal torsion required to reach the geometry of the CI. Complete analysis of the data was hampered by some unusual observations in nonpolar glasses. In this paper we show that they can be explained by assuming dimer formation, with a formation constant of K(eq) = (4 ± 3) × 10(5) M(-1) at 83 K and ΔH(dim) = 7 ± 2 kcal/mol. A complete analysis of the spectra is presented, and fluorescence quantum yields of the monomer and dimer are reported. Efficient self-quenching is found, with a Stern Volmer constant, K(SV) = (1.5 ± 0.1) × 10(6) M(-1), assigned to static quenching. The dimer absorption spectrum was extracted from the data and is compared to Kasha's exciton model and to quantum chemical (QC) calculations. The basic features of exciton splitting are reproduced by quantum mechanical calculations, but complete quantitative agreement of the QC computations with the experimental results is not attained. The previous analysis of the monomer spectra using the displaced harmonic oscillator model is extended to the more demanding conditions prevailing at cryogenic temperatures. The derived ΔH(dim) is in good agreement with other dimers formation enthalpies and with the quantum mechanical calculation presented. The new analysis corrects τ(f) in MCHIP to 2.9 × 10(-13) s, somewhat smaller than the value reported in polar solvent in a previous communication, thereby strengthening the assumption that polarity can reduce the efficiency of CI.

Stability of polynitrogen compounds: the importance of separating the sigma and pi electron systems.

Planar N(x) systems such as cyclo-N(5)(-) and N(5)(+) tend to be more stable than nonplanar systems such as the neutral cyclo-N(6). It is proposed that the key to stabilization is the separation of the sigma and pi electron systems. In both cyclo-N(5)(-) and N(5)(+), a six-pi-electron system is created upon either adding to or removing from the cyclo-N(5) radical one electron. Judicious addition of oxygen atoms to polynitrogen ring compounds such as cyclo-N(4) and cyclo-N(6) can increase their thermodynamic and kinetic stabilities, accompanied by only a small reduction in their efficiency as high energy density materials (HEDMs). The properties of some of these compounds are calculated and compared with the parent all-nitrogen compounds. Coordination of one or more oxygen atoms to the ring leads to effective separation of the sigma and pi electron systems helping to stabilize the systems. Natural bond analysis indicates that the exocyclic NO bonds can assume a single or double bond character, depending on the ring system.

Properties of stable organic bond-stretched non-Lewis molecules.

The conditions required for the existence of a stable bond-stretched singlet isomer of hetero derivatives of bicyclo[2.1.0]pentane (which is a cyclopentane-1,3-diyl derivative) are discussed. Such species are non-Lewis systems with a ruptured C-C bond (formally diradicals), in which two electrons occupy the nonbonding orbital. A high-level calculation shows that in contrast with the carbon substituted compounds, in which the open form is a transition state between two classical-bonded closed bicyclic forms, in the heterosubstituted molecules, the open form is calculated to be a stable minimum. The ionization potentials of the open forms are considerably lower than those of their bicyclic isomers and also of regular organic radicals/diradicals. Nitrogen atoms are found to be more effective than oxygen or sulfur in stabilizing the open isomer. In this case, the open isomer is calculated to be a little more stable than the bicyclic compound, and a barrier of approximately 40 kcal/mol is computed for the ring closing reaction. Thus, the open isomer is both thermodynamically and kinetically stable. This result rationalizes some experimental observations that indicated the existence of non-Lewis singlet species.

Photophysics of (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP): an experimental test for conical intersections.

Fluorescence experiments on (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP) are reported in liquid and glassy solutions. The data indicate a fast decay in the fluid nonpolar, nonprotic solutions (decay times approximately 10(-12) s) and rapid but considerably slower decay in polar ones. In frozen solutions (polar and nonpolar), the fluorescence quantum yield is much higher (near 0.5 and around 0.1 in polar and nonpolar glasses, respectively). The rapid nonradiative transitions in fluid solutions are assigned to internal conversion in both solvent classes, as intersystem crossing is much slower and no net reaction is observed. These results are in agreement with predictions made for the closely related (in terms of electronic structure) but simpler molecule cyclopentadienyl-1,4-dihydropyridine (CPDHP) for which an S1/S0 conical intersection was recently proposed [Int. J. Quant. Chem. 2005, 102, 961]. The crossing of the two lowest singlet states is calculated to vanish in polar solvents such as methyl cyanide, leading to longer lifetime of S1 of CPDHP. As BIDP has a very similar electronic structure, the model predicts a corresponding change in this larger molecule. The strong fluorescence observed in the glassy environments is rationalized by the hindering of the internal torsion required to reach the geometry of the conical intersection.

Biradicals stabilized by intramolecular charge transfer: properties of heterosubstituted pentalene and cyclooctatetraene biradicals.

Intramolecular charge transfer can lead to substantial stabilization of singlet ground state and a corresponding increase of the singlet-triplet gap for molecules isoelectronic with the dianions of antiaromatic hydrocarbons. The formal biradicals 2,5-di-heterosubstituted-pentalenes and 1,5-di-heterosubstituted-cyclooctatetraenes are theoretically predicted to have the potential to be stable, persistent non-Kekulé molecules, as supported by high-level quantum chemical calculations. The singlet-triplet energy gaps and the S(0)-S(1) excitation energies of these molecules are similar to those of aromatic molecules rather than standard biradicals. These formal biradicals have a pronounced zwitterionic character, having a singlet ground state. The marked stabilization of the ground-state singlet for these non-Kekulé molecules is accompanied by a significant destabilization of the highest occupied molecular orbital (HOMO), leading to a low ionization potential (IP). This apparent inconsistency is explained by analyzing the electronic structure of the molecules. In the case of di-aza-pentalene, the energy of the first electronic excited state is only slightly lower than the ionization potential, making it a candidate for molecular autoionization.

The electronic origin of the dual fluorescence in donor-acceptor substituted benzene derivatives.

The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the pi-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures.

Laser ablation of NaN3 and CsN3.

Solid sodium azide and cesium azide crystals were irradiated by high power laser pulses; the ablation products were rapidly cooled by a supersonic expansion of helium and detected by a time of flight mass spectrometer. Neutral and positively charged species were separately recorded and analyzed using 15N isotopomers to help in their assignment. Cluster series of the sequences Na(NaN3)n [or Cs(CsN3)n] were observed, as well as clusters containing NaOH and NaCN; the origin of the C, H, and O atoms appears to be water and CO2 occluded in the salt. Addition of D2O increased the intensity of large clusters and added deuterated ones, whereas addition of chloroform leads to formation of clusters of a Na atom with (NaCl)n clusters. Possible mechanisms for the formation of these clusters are discussed.

Charge separation in ground-state 1,2,4,5-tetra-substituted benzene derivatives.

The conditions required for a formal biradical to exist in a zwitterionic form in the ground state are discussed following the recent experimental observation of zwitterionic structure in the ground state of a quinoid molecule (di-tert-butyl derivative of 2,5-diamino-1,4-benzoquinonediimine, I). A unique characteristic of molecules of this class is the fact that they may be considered as being formed by the union of two radicals, each having an odd number of pi electrons. In the case of I, one fragment carries the two amino group having 7 pi electrons; it acts as the electron donor. The other fragment carries the two oxygen atoms (carrying 5 pi electrons) and acts as an electron acceptor. A model that predicts the properties of these systems is presented, based on previous work on non-Kekule hydrocarbons(2,3) and on the electron donating and attracting properties of the donor and acceptor groups, respectively. The zwitterion is formed by an electron transfer leading to two subunits carrying 6 pi electrons each and may become more stable than the triplet biradical even in the gas phase (i.e., in the absence of an external field) if the ionization potential of the donor is small (of the order of 3-4 eV). In some cases solvation in a polar solvent is required to make the zwitterionic form the lowest energy species on the ground-state surface. The 'spacer' between the donor and acceptor groups (which need not be necessarily derived from an aromatic structure) can be varied and influences the overall dipole moment that is calculated in some cases to be quite large (over 20 D in the gas phase).

Photochemical alpha-cleavage of ketones: revisiting acetone.

The photochemical [small alpha]-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy. In contrast, the decay time of the excited singlet (S(1), (1)n pi) in the bulk is around 10(-9) s and independent of excitation wavelength. Initial excitation to the (1)npi state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal x mol(-1) above the origin of the S(0)-S(1) transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S(1). The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH(3)(*)+ CH(3)CO(*). Direct reaction from the S(1) is negligible, and the non-quenchable reaction (by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10(-12) s or less. The singlet-state decay time measured in the bulk (approximately 1-2 ns) arises from collision-induced processes that populate low-lying levels of S(1). The analysis is aided by detailed state-resolved studies on related molecules (in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.

Electronic degeneracies in symmetric (Jahn-Teller) and nonsymmetric aliphatic radical cations: global topology of sigma-bonded molecules.

The ground-state potential surfaces of five aliphatic radical cations are investigated using a spin-pairing model. It is shown that the ground-state surface of an n-atomic system supports several stationary points (minima and transition states, including second-order ones). In addition, there are numerous nuclear configurations at which the ground state is electronically degenerate. The electronic degeneracies due to interactions between atoms bound to the same atom are either 2-fold (conical intersections) or 3-fold degenerate but not of a higher dimension. Each 3-fold degeneracy is accompanied by an even number of conical intersections (four or two). A systematic procedure for locating all of these nuclear configurations (that are in fact 3n - 8 or 3n - 11 dimensional hypersurfaces) is described. The model allows for the qualitative determination of the structure and charge distribution of the system at all of the stationary points and electronic degeneracies. Quantum chemical calculations confirm the predictions of the model, which is used to direct and facilitate the calculations.

Isomerization around a C=N double bond and a C=C double bond with a nitrogen atom attached: thermal and photochemical routes.

The Longuet-Higgins phase change theorem is used to show that, in certain photochemical reactions, a single product is formed via a conical intersection. The cis-trans isomerization around the double bond in the formaldiminium cation and vinylamine are shown to be possible examples. This situation is expected to hold when the reactant can be converted to the product via two distinct elementary ground-state reactions that differ in their phase characteristics. In one, the total electronic wavefunction preserves its phase in the reaction; in the other, the phase is inverted. Under these conditions, a conical intersection necessarily connects the first electronic excited state to the ground state, leading to rapid photochemical isomerization following optical excitation. Detailed quantum chemical calculations support the proposed model. The possibility that a similar mechanism is operative in other systems, among them the rapid photo-induced cis-trans isomerization of longer protonated Schiff bases (the parent chromophores of rhodopsins), is discussed.

A valence bond analysis of electronic degeneracies in Jahn-Teller systems: low-lying states of the cyclopentadienyl radical and cation.

The lowest doublet electronic state of the cyclopentadienyl radical (CPDR) and the lowest singlet state of the cyclopentadienyl cation (CPDC) are distorted from the highly symmetric D(5h) structure due to the Jahn-Teller effect. A valence bond analysis based on the phase-change rule of Longuet-Higgins reveals that in both cases the distortion is due to the first-order Jahn-Teller effect. It is shown that, while for the radical an isolated Jahn-Teller degeneracy is expected, in the case of the cation the main Jahn-Teller degeneracy is accompanied by five satellite degeneracies. The method offers a chemically oriented way for identifying the distortive coordinates.