Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: A (2)Sigma(v(') = 0)<--X (2)Pi(v(") = 0,1,2,3). Simulations of the NO rovibronic intensities for the A<--X transitions show that dissociated NO from RDX and HMX is rotationally cold (approximately 20 K) and vibrationally hot (approximately 1800 K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.     

On the excited electronic state dissociation of nitramine energetic materials and model systems

In order to elucidate the difference between nitramine energetic materials, such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), and their nonenergetic model systems, including 1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine, and dimethylnitramine, both nanosecond mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the UV spectral region have been employed to investigate the mechanisms and dynamics of the excited electronic state photodissociation of these materials. The NO molecule is an initial decomposition product of all systems. The NO molecule from the decomposition of energetic materials displays cold rotational and hot vibrational spectral structures. Conversely, the NO molecule from the decomposition of model systems shows relatively hot rotational and cold vibrational spectra. In addition, the intensity of the NO ion signal from energetic materials is proportional to the number of nitramine functional groups in the molecule. Based upon experimental observations and theoretical calculations of the potential energy surface for these systems, we suggest that energetic materials dissociate from ground electronic states after internal conversion from their first excited states, and model systems dissociate from their first excited states. In both cases a nitro-nitrite isomerization is suggested to be part of the decomposition mechanism. Parent ions of dimethylnitramine and nitropyrrolidine are observed in femtosecond experiments. All the other molecules generate NO as a decomposition product even in the femtosecond time regime. The dynamics of the formation of the NO product is faster than 180 fs, which is equivalent to the time duration of our laser pulse.     

Ab initio study of a biradical radiationless decay channel of the lowest excited electronic state of cytosine and its derivatives

A theoretical model for the ultrafast S1-->S0 internal conversion of cytosine is presented, in which a state switch from the initially prepared 1pipi* state to the out-of-plane deformed excited state of biradical character controls the rate of the S1(1pipi*) decay. This mechanism successfully accounts for the dramatically longer S1 lifetimes of 5-fluorocytosine and N-acetylcytosine relative to cytosine. The replacement of the C5 hydrogen atom by a methyl group is predicted to lead to a substantial, but not dramatic, increase in the S1 lifetime, also consistent with experiment. It is this ability to correctly predict the substituent effects that distinguishes the present model from the previously proposed mechanisms.     

Spectroscopy of highly excited vibrational states of HCN in its ground electronic state

An experimental technique based on a scheme of vibrationally mediated photodissociation has been developed and applied to the spectroscopic study of highly excited vibrational states in HCN, with energies between 29,000 and 30,000 cm(-1). The technique consists of four sequential steps: in the first one, a high power laser is used to vibrationally excite the sample to an intermediate state, typically (0,0,4), the nu3 mode being approximately equivalent to the C-H stretching vibration. Then a second laser is used to search for transitions between this intermediate state and highly vibrationally excited states. When one of these transitions is found, HCN molecules are transferred to a highly excited vibrational state. Third, a ultraviolet laser photodissociates the highly excited molecules to produce H and CN radicals in its A 2Pi electronic state. Finally, a fourth laser (probe) detects the presence of the CN(A) photofragments by means of an A-->B-->X laser induced fluorescence scheme. The spectra obtained with this technique, consisting of several rotationally resolved vibrational bands, have been analyzed. The positions and rotational parameters of the states observed are presented and compared with the results of a state-of-the-art variational calculation.     

Resonance Raman spectra of β-carotene and its molecular structure in the excited electronic state

The resonance Raman spectra of β-carotene have been obtained at low temperature. The excitation profiles of ν₁ (1525 cm^?1) and 2ν₁ (3043 cm^?1) are analysed in terms of the Albrecht theory. The overlap integrals between the vibrational wavefunctions of the ground and the first excited electronic states are shown to be the most important factor in determining the resonance Raman intensities of this molecule. Information on the structure of the electronically excited state has been obtained.     

Molecular structure,conformational stability,energetic and intramolecular hydrogen bonding in ground,and electronic excited state of 3-mercapto propeneselenal

In the present work, a conformational analysis of 3-mercapto propeneselenal is performed using several computational methods, including DFT (B3LYP), MP2, and G2MP2. At the DFT and G2MP2 levels the most stable conformers of title compound are characterized by an extended backbone structure, minimizing the steric repulsions between the sulfur and selenium lone pairs. Two conformers exhibit hydrogen bonding. This feature, although not being the dominant factor in energetic terms, appears to be of foremost importance to define the geometry of the molecule. The influence of the solvent on the stability order of conformers and the strength of intramolecular hydrogen bonding was considered using the PCM, SCI–PCM, and IEF–PCM methods. The results of analysis by quantum theory of “Atoms in Molecules” and natural bond orbital method fairly support the DFT results. The calculated HOMO and LUMO energies showed that charge transfer occurs within the molecule. Further verification of the obtained transition state structures was implemented via intrinsic reaction coordinate analysis. Calculations of the ¹H NMR chemical shift at GIAO/B3LYP/6–311++G** levels of theory are also presented. The excited-state properties of intramolecular hydrogen bonding in hydrogen-bonded systems have been investigated theoretically using the time-dependent density functional theory method.     

Ground state and excited state dipole moments of alkyl substituted para-nitroaniline derivatives

The ground state and excited state dipole moment of a series of alkyl substituted para-nitroaniline derivatives is reported. Ground state dipole moment was determined by the Debye-Guggenheim method and the excited state dipole moment was estimated using the solvatochromic method. For all molecules under investigation, the excited state dipole moment was found to be higher than the ground state dipole moment. The molecules exhibited positive solvatochromism.     

Hidden electronic excited state of enhanced green fluorescent protein

Precise two-photon absorption spectra of the green fluorescent protein (GFP) and the mutants sapphire-GFP (T203I) and enhanced GFP (S65T/F64L), as well as a model compound for the chromophore, 4'-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) were measured by multiplex two-photon absorption spectroscopy. The observed TPA bands of the anionic forms of enhanced GFP and HBDI were significantly shifted to the higher energy compared with the lowest-energy bands in one-photon absorption spectra. This result indicated the existence of a hidden electronic excited state in the vicinity of the lowest excited singlet (S1) state of the anionic form of the GFP chromophore, which is the origin of the blue shift of the two-photon absorption spectra as well as two-photon fluorescence excitation spectra.     

Quantum chemical study of the electronic structure of NiCH2 + in its ground state and low-lying electronic excited states

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.     

The electronic spectra and excited state dipole moment of 2-fluoropyridine

The electronic absorption spectrum in the vapour state and in solution in different solvents in the region 3000–1900 Å and the fluorescence and phosphorescence emission spectra in ethanol or cyclohexane at 77 K have been studied for 2-fluoropyridine and analysed. Two systems of absorption band corresponding to the π→π* transition II and π→π* transition III have been observed and the excited state dipole moments have been determined from the solvent-induced shifts of the electronic absorption bands. The half-life of phosphorescence in cyclohexane at 77 K is found to be 3.5 s.     

Multicoordinational excited state twisting of indan-1,3-dione derivatives

Excited state relaxation of indan-1,3-dione derivatives with different substituents attached to the phenyl ring and with the bridged amino group was investigated by means of the steady-state fluorescence and femtosecond time-resolved absorption pump–probe spectroscopy. Bridging of the amino group increases the fluorescence quantum yield and the excited state lifetime. Analysis of the results indicates that the phenyl ring twisting around a single central bond leads to the nonradiative state formation and to subsequent fast relaxation to the ground state. Double bond twisting takes place in molecules with the bridged amino group and causes a large Stokes shift and slightly slower excited state relaxation.     

Role of excited state intramolecular charge transfer in the photophysical properties of norfloxacin and its derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.     

Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.     

Chiral synthetic pseudopeptidic derivatives as triplet excited state quenchers

The behavior of 6 pseudopeptidic models, synthesized by connecting different protected amino acids (Trp, Tyr, Phe, and Lys) with various diamino spacers, as quenchers of the triplet excited state of tiaprofenic acid (and its methyl ester), has been investigated. A series of quenching constants have been determined, which depend on the nature of the quencher and on the stereochemistry of the excited drug. A significant degree of stereodifferentiation has been found for the peptidomimetic synthesized with Phe and Tyr linked by a piperazine bridge. The obtained results support the utility of laser flash photolysis (LFP) as a tool to investigate the interactions between photoexcited drugs and simple models of binding sites in proteins.     

2-Aminopurine excited state electronic structure measured by stark spectroscopy

2-Aminopurine (2AP) is an adenine analogue that has a high fluorescence quantum yield. Its fluorescence yield decreases significantly when the base is incorporated into DNA, making it a very useful real-time probe of DNA structure. However, the basic mechanism underlying 2AP fluorescence quenching by base stacking is not well understood. A critical element in approaching this problem is obtaining an understanding of the electronic structure of the excited state. We have explored the excited state properties of 2AP and 2-amino,9-methylpurine (2A9MP) in frozen solutions using Stark spectroscopy. The experimental data were correlated with high level ab initio (MRCI) calculations of the dipole moments, mu0 and mu1, of the ground and excited states. The magnitude and direction of the dipole moment change, Deltamu01 = mu1 - mu0, of the lowest energy optically allowed transition was determined. While other studies have reported on the magnitude of the dipole moment change, we believe that this is the first report of the direction of Deltamu, a quantity that will be of great value in interpreting absorption spectral changes of the 2AP chromophore. Polarizability changes due to the transition were also obtained.     

Evidence for excited electronic state interference in resonance raman scattering

The complex resonance Raman spectra of molecular bromine have been analyzed quantitatively and a clear demonstration of interference in the Raman intensity from the B(³11₀⁺_u) and ¹17_1u excited states has been found.     

The electronic absorption spectrum and excited state dipole moment of 3-fluoropyridine

The electronic absorption spectrum of 3-fluoropyridine in the vapour state and in solutions in different solvents in the region 3000-1900 Å has been measured and analysed. Three systems of absorption bands; n→π* transition I, π→π* transition II and π→π* transition III are identified. The oscillator strength of the absorption band systems due to the π→π* transition II and π→π* transition III and the excited state dipole moments associated with these transitions have been determined by the solvent-shift method.     

Hydration-dependent structural deformation of guanine in the electronic singlet excited state

Theoretical study was performed to investigate how the degree of hydration affects the structures and properties of the canonical form (keto-N9H) of guanine in the ground and lowest singlet pipi* excited state. This work is the continuation of our earlier work where we have studied the hydration of guanine in the first solvation shell with one, three, five, and six water molecules. In the present investigation, we have considered 7-13 water molecules in hydrating guanine. Ground-state geometries were optimized at the Hartree-Fock level, whereas the configuration interaction-singles (CIS) method was used for the excited-state geometry optimization. The 6-311G(d,p) basis set was used in all calculations. The harmonic vibrational frequency analysis was used to determine the nature of the optimized ground- and excited-state potential energy surfaces; all geometries were found to be minima at the respective potential surfaces. It was found that the degree of hydration has a significant influence on the excited-state structural nonplanarity of guanine. It is expected that excited-state dynamics of guanine will depend on the degree of hydration. Ground- and excited-state geometries of selected hydrated species were also optimized in the bulk water solution using the polarizable continuum model (PCM). It was found that bulk water solution generally does not have significant influence on the structure of the hydrated species. Effects of hydration on different stretching vibrations in the ground and excited states are also discussed.     

Ultrafast dynamics and excited state deactivation of [Ru(bpy)2Sq]+ and its derivatives

Femtosecond transient absorption spectroscopy has been employed to understand the excited state dynamics of [Ru(bpy)(2)Sq](+) (I; bpy is 2,2'-bipyridyl, and Sq is the deprotonated species of the semiquinone form of 1,2-dihydroxy benzene) and its derivatives, a widely studied near-infrared (NIR) active electrochromic dye. Apart from the well-defined dpi(Ru) --> pi(bpy)-based metal-to-ligand charge transfer (MLCT) transition bands at approximately 480 nm, this class of molecules generally shows another dpi(Ru) --> pi(Sq)(SOMO)-based intense MLCT band at around 900 nm, which is known to be redox active and bleaches reversibly upon a change in the oxidation state of the coordinated dioxolene moiety. To have better insight into the photoinduced electron transfer dynamics associated with this MLCT transition, detailed investigations have been carried out on exciting this MLCT band at 800 nm. Immediately after photoexcitation, bleach at 900 nm has been observed, whose recovery is found to follow a triexponential function with major contribution from the ultrafast component. This ultrafast component of approximately 220 fs has been ascribed to the S(1) to S(0) internal conversion process. In addition to the bleach, we have detected two transient species absorbing at 730 and 1000 nm with a formation time approximately 220 fs for both species. The excited state lifetimes for these two transient species have been measured to be 1.5 and 11 ps and have been attributed to excited singlet ((1)MLCT) and triplet ((3)MLCT) states, respectively. Transient measurements carried out on the different but analogous derivatives (II and III) have also shown similar recovery dynamics except that the rate for the internal conversion process has increased with the decrease in the S(1) to S(0) energy gap. The observed results are consistent with the energy gap law for nonradiative decay from S(1) to S(0).