The control of the structure and optical properties of liquid-crystalline (LC) phases by means of light is a major challenge in the development of molecular device and optical data storage systems. [1] For instance the use of photochromic organic molecules for the modulation of mesophases is illustrated by the switching between nematic and isotropic LC phases based on the photochemical cis±trans isomerization of doped azobenzene in nematic media. [2] It is well known that nematic liquid crystals can be converted into chiral nematic (induced cholesteric) LC materials using a chiral dopant. [3] Since nematic and chiral nematic LC phases exhibit distinct optical properties photochemical switching between these phases is highly significant for display technology. In a few cases reversible optical switching between a nematic (N) and chiral nematic (N*) phase has recently been demonstrated. [4±8] We have reported the photomodulation of LC phases using ªchiroptical molecular switchesº. [5,8] Photochemical interconversion of two opposite helices of cis and trans isomers of overcrowded alkenes doped in nematic LCs resulted in switching between two cholesteric phases, depending on the irradiation wavelength employed. As the isomers of the chiral dopant were pseudoenantiomers the pitches of the two phases were opposite in sign. [5] When enantiomers were used instead (racemic mixture), circular polarized light was necessary in order to achieve a small enantiomeric excess, which was amplified by doping the chiral overcrowded alkene photoswitch in a nematic LC phase, which resulted in a chiral nematic phase. [8] It is also conceivable that switching between nematic and cholesteric phases can be achieved by doping with a chiral photoswitch that exhibits large difference in twisting power in its two photostates. [4±7] By doping the nematic LC sample with the amount of dopant needed for the formation of a chiral liquid-crystalline phase only in one photostate a N ® N* or N* ® N LC switch can be produced.Diarylethenes were found to be efficient optical switches (Scheme 1) [9±13] and the two photostates show very different absorption spectra (depending on the substituents). The absorption maximum of the closed-form diarylethene is about 300 nm red shifted from the absorption maximum of the open form, a feature that was used as a non-destructive read-out method. Chiral diarylethenes were shown to undergo asymmetric cyclization, providing one diastereo-_______________________ ± [*] Prof.

[reaction: see text] TATP gives rise to two separable conformations because the barrier for interconversion between them is relatively high at room temperature. This kind of behavior is rare in cyclic organic systems and is the result of poor overlap in the "flip-flop" transition state. The crystal structure of the analogous tricyclohexanone triperoxide also indicates the presence of two conformers.

Structural, protecting group and leaving group effects in the formation of oxocarbenium intermediates were studied in the gas phase. It is found that significant stabilization of oxocarbenium cations is achieved by protecting groups that interact with the cationic center via neighboring group participation despite the electron-withdrawing character of these moieties. On the other hand, ethereal protecting groups do not facilitate the formation of oxocarbenium intermediates. The experimental findings are supported by DFT calculations that show the following order of stabilization by the group adjacent to the cationic center: RCO > SiR(3) > R, where R is an alkyl group. This indicates that the SN1-like mechanism that is commonly proposed for this reaction is not always valid. Moderate leaving group effect is also detected in a series of thioaryl glucopyranosides.

Collision-induced dissociation of ammonium-cationized alpha and beta acetyl pyranosidic isomers were studied and stereochemical dependence of the reactivity towards elimination of acetic acid from the anomeric position was found. It is shown that isomers that contain trans diacetyloxy groups at positions 1 and 2 of the pyranoside are more reactive, allowing anomeric distinction according to the relative abundance of the oxocarbenium product ion of this reaction in the spectrum. The higher reactivity of trans isomers is rationalized by neighboring group assistance that is possible only in the trans configuration. DFT calculations indicate that the lesser energetic reaction path occurs in an ammonium-cationized trans diequatorial 2,3-diacetoxy tetrahydropyran that was used as a model in order to study this process theoretically. It is also found that the configuration at position 4 of the carbohydrate plays a major role in the rate of formation and stability of oxocarbenium ions.

A new high performance yet insensitive explosive is proposed, based on an extensive computational study of tris(tetrazolyl)amine in the solid state and in the gas phase. The calculations for the solid state employ the PBE density functional with empirical dispersion correction while the gas phase calculations use the higher level of B3LYP and MP2. Two stable crystalline structures of tris(tertrazolyl)amine were located belonging to P1 and P21 space groups. The crystal structures obtained reveal that solid tris(tertrazolyl)amine is organized in layers with a very small interlayer spacing, due to π stacking, as well as significant inter and intra-molecular hydrogen bonding. Application of these results to the calculation of the detonation velocity and pressure indicate high performance for both phases, especially the P21 phase. At the same time the small value found for the interlayer spacing and the significant hydrogen bonding suggest relatively high stability. These results point to a promising new explosive material with a balance between high performance and insensitivity.

Changing the state: The electrophilicity of benzhydryl cations (see formula) towards neutral amine nucleophiles in the gas phase was determined. It was found that the kinetics of electrophile–nucleophile gas‐phase reactions correlate well with values known in solution although the actual electrophilicity parameters are considerably smaller in the gas phase.

In this study we report on high‐energy, collision‐induced dissociation processes leading to charge‐remote fragmentations, using three alkyl cations, namely n‐hexadecylpyridinium, n‐hexadecyltriphenylphosphonium and n‐hexadecyltriethylammonium, each with and without 2H2‐labelling at the C(9) position of the hexadecyl chain. The characteristic patterns corresponding to the formal elimination of alkane elements were observed, and the 2H2‐labelling at C(9) clearly affected only one charge‐remote fragment ion of the homologous series. However, in addition to the expected fragment ion containing only one deuterium atom, a significant ion retaining two deuterium atoms was observed. MS/MS/MS experiments demonstrated clearly that the latter ion showed partial deuteration around the charge site, the level of deuteration depending on the structure of the original precursor cation. These results can be interpreted in terms of two novel, distinct mechanisms, one of which involves an excited state in an aromatic ring. Mixed‐site fragmentation (MSF) ions were also observed from the phosphonium and ammonium ion precursors. We believe that the observation of the MSF process occurring at an sp2‐hybridized center in the phosphonium series has not been reported previously. It thus becomes apparent that high‐energy collisions leading to charge‐remote reactions in fact lead to a broad range of pathways. © 2000 John Wiley & Sons, Ltd.

The favored fragmentation pathway for protonated and alkylated pyridinium cations of the general formula p-XC 6 H 4 CH 2 CH 2 CH¢CH Py ϩ R (R¢H, Me; Py¢pyridine) is a C™C homolytic cleavage. The tendency to form radicals is higher for alkylated pyridinium cations than for the protonated ones that can also afford closed-shell products. Theoretical calculations show that the singlet-triplet gap for transient structures with an elongated benzylic C™C bond is very low and the formation of radicals may result from mixing of these states. In addition to the notable substituent effect on the fragmentation efficiency of the cations under study, calculated results show a clear substituent effect on the singlet-triplet transitions. We also observe that triphenylphosphonium cations behave notably different. Thus, the pyridinium system that contains a p-chloro benzyl moiety loses a benzyl radical readily while the analogous triphenylphosphonium cation is very stable under the same conditions. O rganic closed shell ions typically dissociate into closed shell ionic and neutral products. Nevertheless, there are exceptions to this rule that may result from lack of selectivity under extremely energetic conditions, unusual stability of some cation radicals or specific kinetic effects. It is reasonable that highly energetic conditions promote simple bond cleavage, either heterolytic or homolytic. Some radicals, however are generated under mild conditions and their formation can only be attributed to their relative stability.Different classes of organic closed-shell ions undergo C™C and C™N cleavages that are not necessarily charge driven [1][2][3][4][5]. Furthermore, it has been suggested that these reactions occur through a concerted mechanism or are mediated by the formation of radicals [6 -9]. The development of electrospray ionization allowed the formation of various classes of closed shell ions under very mild conditions and hence it has been unambiguously shown that at least in some cases homolytic C™C and C™H cleavages are involved in the gas phase decomposition of cationized organic compounds [10,11].One class of organic cations that are known to undergo homolytic cleavages is quaternary ammonium and pyridium cations [8,[12][13][14][15][16][17]. Fisher and Veith studied the gas phase fragmentation of ammonium and pyridinium ions, generated with field desorption and reported the formation of nitrogen based radical cations under collision induced dissociation (CID) [12,13]. Harrison showed that the formation of radicals and cations from quaternary ammonium ions is dominated by the relative ionization potential of the appropriate radicals and neutrals, showing that the reaction products are the ones thermodynamically favored [14,15]. Katritzky and coworkers studied homolytic cleavages in pyridium cations that were designed in order to show the substituent effect on the relative stability of radicals. They propose the term "merostabilization" to describe stabilization that results from the presence of both an electron-attracti...