Stuart A. Hayes

On the Molecular and Electronic Structures of AsP3 and P4

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.

Direct observation of collective modes coupled to molecular orbital-driven charge transfer

The making of a molecular movie Phase transitions familiar from everyday life, such as boiling or melting, are caused by changing the temperature. In the laboratory, however, researchers can also change the phase of a material by shining intense light on it. During such transitions, changes occur in both the electronic and lattice structure of the material. Ishikawa et al. used ultrafast optical and electron diffraction probes to monitor both types of change simultaneously during a photo-induced phase transition in a molecular crystal. The resulting molecular movies showed expansion of the intermolecular distance, flattening of the molecules, and tilting of molecular dimers. Science, this issue p. 1501 Ultrafast spectroscopy and electron diffraction are used to create molecular movies of a phase transition in Me4P[Pt(dmit)2]2. Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

N,N‐Dimethylaminopropylsilane: A Case Study on the Nature of Weak Intramolecular Si⋅⋅⋅N Interactions

N,N-Dimethylaminopropylsilane H(3)Si(CH(2))(3)NMe(2) was synthesised by the reaction of (MeO)(3)Si(CH(2))(3)NMe(2) with lithium aluminium hydride. Its solid-state structure was determined by X-ray diffraction, which revealed a five-membered ring with an SiN distance of 2.712(2) A. Investigation of the structure by gas-phase electron diffraction (GED), ab initio and density functional calculations and IR spectroscopy revealed that the situation in the gas phase is more complicated, with at least four conformers present in appreciable quantities. Infrared spectra indicated a possible SiN interaction in the Si-H stretching region (2000-2200 cm(-1)), as the approach of the nitrogen atom in the five-membered ring weakens the bond to the hydrogen atom in the trans position. Simulated gas-phase IR spectra generated from ab initio calculations (MP2/TZVPP) exhibited good agreement with the experimental spectrum. A method is proposed by which the fraction of the conformer with a five-membered ring can be determined by a least-squares fit of the calculated to experimental absorption intensities. The abundance of this conformer was determined as 23.7(6) %, in good agreement with the GED value of 24(6) %. The equilibrium SiN distance predicted by theory for the gas-phase structure was highly variable, ranging from 2.73 (MP2) to 3.15 A (HF). The value obtained by GED is 2.91(4) A, which could be confirmed by a scan of the potential-energy surface at the DF-LCCSD[T] level of theory. The nature of the weak dative bond in H(3)Si(CH(2))(3)NMe(2) can be described in terms of attractive inter-electronic correlation forces (dispersion) and is also interpreted in terms of the topology of the electron density.

Cold ablation driven by localized forces in alkali halides

Laser ablation has been widely used for a variety of applications. Since the mechanisms for ablation are strongly dependent on the photoexcitation level, so called cold material processing has relied on the use of high-peak-power laser fluences for which nonthermal processes become dominant; often reaching the universal threshold for plasma formation of ~1 J cm(-2) in most solids. Here we show single-shot time-resolved femtosecond electron diffraction, femtosecond optical reflectivity and ion detection experiments to study the evolution of the ablation process that follows femtosecond 400 nm laser excitation in crystalline sodium chloride, caesium iodide and potassium iodide. The phenomenon in this class of materials occurs well below the threshold for plasma formation and even below the melting point. The results reveal fast electronic and localized structural changes that lead to the ejection of particulates and the formation of micron-deep craters, reflecting the very nature of the strong repulsive forces at play.

Variations in the Mechanisms of Direct Metallation of Cyclic and Acyclic Aminals

The development of preparative protocols for direct ametallation of amines is a challenging but worthwhile task, due to the importance of such reagents for synthesis. Because of the electron-density accumulation on the adjacent nitrogen atom and carbanionic function, these systems are regarded as non-stabilised or even destabilised carbanions. Alternative multi-step approaches to a-metallated amines via transmetallation, reactions of iminium salts with lowvalent metal halides, C S or C Te bond cleavage or activation of amines by BF3 adduct formation [5] involve enormous preparative effort and are inefficient due to loss of substance. Despite substantial progress in the few last years, the number of amine systems directly accessible to deprotonation is still easy to overlook. n-Butyllithium mono-lithiates N,N ,N -trimethyl-1,4,7-triazacyclononane at one methyl group; N-methylpiperidine is deprotonated at the CH3 group using Schlosser s base; the deprotonation of Me2NACHTUNGTRENNUNG(CH2)2NMe2 (TMEDA) was described by two groups, but never occurs in large yield; MeNACHTUNGTRENNUNG[(CH2)2NMe2]2 (PMDTA) can be lithiated, but again in limited yield; and the chiral diamine ((R,R)-tetramethyl-1,2-diaminocyclohexane ((R,R)TMCDA) was found to undergo direct metallation—again at its terminal CH3 group. [10] The latter three examples are important due to the fact that the substrates are widely applied as auxiliary bases in alkyllithium chemistry, without the intention of metallating these. Generating a carbanion adjacent to two nitrogen atoms seemed even more difficult. However, we recently found that aminal units incorporated in saturated six-membered rings are selectively lithiated at the endocyclic NCH2N position. The reaction of 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) with nBuLi and even better with tBuLi in hexane proceeds smoothly giving a product consisting of a dimer of lithiated TMTAC linked into chains alternating with TMTAC, [{MeN ACHTUNGTRENNUNG[CH2N(Me)]2CHLi}2·TMTAC]1.[11] Later we found that conducting the reaction for a longer time at low temperatures leads to the completely lithiated compound. A similar result was found for 1,3-dimethyl-1,3-diazacyclohexane (DMDAC), which is also lithiated at the NCH2N position by tBuLi. These compounds have been shown to be applicable as Corey–Seebach lithiated dithiane analogous reagents, but with the advantage of heavy-metal free workup procedures. Even a system that has two diazacyclohexane units joined via a methylene group H2CACHTUNGTRENNUNG[NCH2N(Me)ACHTUNGTRENNUNG(CH2)3]2 can be doubly deprotonated in high yields to give a dimer of H2C ACHTUNGTRENNUNG[NCH(Li)N(Me) ACHTUNGTRENNUNG(CH2)3]2.[13] These results are in contrast to reports by Karsch, in which open-chain aminals of the type RMeNCH2NMeR can be simultaneously lithiated at both methyl groups by treatment with tBuLi (Scheme 1). Obviously there are different mechanisms involved.

An Improved Gas Electron Diffractometer - The Instrument, Data Collection, Reduction and Structure Refinement Procedures

Graphical Abstract An Improved Gas Electron Diffractometer – The Instrument, Data Collection, Reduction and Structure Refinement Procedures

The perfluorinated alcohols (F5C6)(F3C)2COH and (F5C6)(F10C5)COH: synthesis, theoretical and acidity studies, spectroscopy and structures in the solid state and the gas phase

The syntheses of the perfluorinated alcohols (F(5)C(6))(F(3)C)(2)COH (1) and (F(5)C(6))(C(5)F(10))COH (2) are described. Both compounds were prepared in reasonable yields (1: 65%, 2: 85%) by reacting the corresponding ketone with C(6)F(5)MgBr, followed by acidic work-up. The alcohols were characterized by NMR, vibrational spectroscopy, single-crystal X-ray diffraction, acidity measurements and gas-phase electron diffraction. A combination of appropriate 2D NMR experiments allowed the unambiguous assignment of all signals in the (19)F spin systems, of which that of 2 was especially complex. High acidity of the alcohols is indicated by acidity measurements as well as the calculated gas phase acidities. It is also supported by the crystal structure of 2, which exhibits only a single weak intermolecular hydrogen bridge with an O...O distance of 301 pm. This shows the low donor strength of the oxygen atom in the compound, which is partly compensated through formation of two intramolecular CF...H contacts of 220 and 232 pm length to the proton not involved in the hydrogen bridge. The pK(a) values in acetonitrile are 22.2 for 1 and 22.0 for 2; their calculated gas phase acidities are 1367 and 1343 kJ mol(-1) (MP2/TZVPP level).

Chlorobis(pentafluoroethyl)phosphane: Improved Synthesis and Molecular Structure in the Gas Phase

(C(2)F(5))(2)PCl is now accessible through a significantly improved synthesis protocol starting from the technical product (C(2)F(5))(3)PF(2). (C(2)F(5))(3)PF(2) was reduced in the first step with NaBH(4) in a solvent-free reaction at 120 °C. The product, P(C(2)F(5))(3), was treated with an excess of an aqueous sodium hydroxide solution to afford the corresponding phosphinite salt Na(+)(C(2)F(5))(2)PO(-) selectively under liberation of pentafluoroethane. Subsequent chlorination with PhPCl(4) resulted in the selective formation of (C(2)F(5))(2)PCl, which was isolated by fractional condensation in an overall yield of 66 %. The gas electron diffraction (GED) pattern for (C(2)F(5))(2)PCl was recorded and found to be described by a two-conformer model. A quantum chemical investigation of the potential-energy surface revealed the possible existence of many low-energy conformers, each with a number of low-frequency vibrational modes and therefore large-amplitude motions. The conformer calculated to be most stable was also found to be most abundant by GED and comprised 61(5) % of the total. The molecular structure parameters determined by GED were in good agreement with those calculated at the MP2/TZVPP level of theory; the only significant difference was a discrepancy of about 3° in the C-P-C angle, which, for the lowest-energy conformer, was refined to 98.2(4)° and was calculated to be 94.9°.

Synchronised photoreversion of spirooxazine ring opening in thin crystals to uncover ultrafast dynamics

Reversibility is an important issue that prevents ultrafast studies of chemical reactions in solid state due to product accumulation. Here we present an approach that makes use of spectrally-selected, post-excitation, ultrashort laser pulses to minimise photoproduct build-up, i.e. recover before destroy. We demonstrate that this method enabled us to probe the ultrafast dynamics of the ring opening reaction of spironaphthooxazine thin crystals by means of transient absorption spectroscopy. By extension, this approach should be amenable to other photochromic systems and use with structural probes.

Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation

Solid-state reactions are influenced by the spatial arrangement of the reactants and the electrostatic environment of the lattice, which may enable lattice-directed chemical dynamics. Unlike the caging imposed by an inert matrix, an active lattice participates in the reaction, however, little evidence of such lattice participation has been gathered on ultrafast timescales due to the irreversibility of solid-state chemical systems. Here, by lowering the temperature to 80 K, we have been able to study the dissociative photochemistry of the triiodide anion (I3-) in single-crystal tetra-n-butylammonium triiodide using broadband transient absorption spectroscopy. We identified the coherently formed tetraiodide radical anion (I4•-) as a reaction intermediate. Its delayed appearance after that of the primary photoproduct, diiodide radical I2•-, indicates that I4•- was formed via a secondary reaction between a dissociated iodine radical (I•) and an adjacent I3-. This chemistry occurs as a result of the intermolecular interaction determined by the crystalline arrangement and is in stark contrast with previous solution studies.