Photofragmentation of Tetranitromethane: Spin-Unrestricted Time

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Letter

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-State Molecular Dynamics Yulun Han, Bakhtiyor Rasulev, and Dmitri S. Kilin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01330 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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The Journal of Physical Chemistry Letters

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Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-

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State Molecular Dynamics

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Yulun Han,†,‡ Bakhtiyor Rasulev,∥ and Dmitri S. Kilin*,†,‡

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†

5

∥Department

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58102, United States

7

‡

8

United States

Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States of Coatings and Polymeric Materials, North Dakota State University, Fargo, North Dakota

Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108,

9 10

Abstract:

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In this study, the photofragmentation dynamics of tetranitromethane (TNM) is explored by a spin-

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unrestricted time-dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi

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oscillations and principles similar to trajectory surface hopping, with a mid-intensity field approximation.

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The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations.

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During excitation cycles, the nuclear kinetic energy is accumulated to overcome the dissociation barriers

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in the reactant and a sequence of intermediates. The dissociation pathway includes the ejection of a NO2

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group followed by the formation of NO and CO. The simulated mass spectra at the ab initio level, based

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on the bond length in possible fragments, are extracted from simulation trajectories. The recently

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developed methodology has the potential to model and monitor photoreactions with open-shell

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intermediates and radicals.

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Tetranitromethane (TNM), designated as C(NO2)4, is an oxygen-rich methane derivative.

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The oxygen weight per unit volume of TNM is nearly the same as that of liquid oxygen itself.1

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TNM is a volatile liquid at ambient conditions with a 399 K normal boiling point and a 287 K

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melting point.1-2 The structure of TNM consists of four equivalent nitro groups around the

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tetrahedral carbon center.2-3 One nitro group of TNM shows great mobility in chemical

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reactions.2,

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propellant.1,

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with hydrocarbons.4 A plant working upon the manufacture of TNM on an industrial scale was

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destroyed by an explosion in 1953.1 There have been many experimental investigations on the

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pyrolysis and photolysis of TNM.3, 8-10 In addition, extensive simulations have been carried out

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to study the thermal or photochemical decomposition of the simplest nitroalkane e.g.

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nitromethane (NM).11-16 For instance, Fileti et al. reported the detonation kinetics and explosion

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reaction of NM computed by reactive force field (ReaxFF) molecular dynamics (MD)

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simulations.11 However, to our knowledge, theoretical studies addressing the photodissociation

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of TNM due to photoinduced electronic transitions are limited.

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TNM has a wide range of applications such as a nitrating reagent and rocket 5-7

TNM becomes a highly sensitive and powerful explosive when contaminated

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The computational description of photoinduced reactions is a great challenge and a

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practical demand. Reduced density matrix (RDM) equation of motion allows analysis of

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photoinduced relaxation of charge carriers for semiconductors.17-19 Nonadiabatic molecular

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dynamics (NAMD) have been used to study photophysical processes, such as charge transfer,

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and photochemical processes, such as photoisomerization and photodissociation, due to

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nonadiabatic dynamics.12, 20-32 The photoluminescence yields of silicon nanocrystals have been

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investigated through NAMD with a multireference treatment of the electronic structure.33 In a

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recent study, the spin-restricted time-dependent excited-state molecule dynamics (TDESMD) 2 ACS Paragon Plus Environment

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was used to simulate the photofragmentation of neutral lanthanide cyclopentadienyl complexes,

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where the computed fragments show good agreement with fragments experimentally observed

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by photoionization time-of-flight (PI-TOF) mass spectrometry.34-36 It should be noted that

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TDESMD is interpreted as “molecular dynamics for time-dependent electronic configuration”.

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This highlights the difference from ground state MD, fixed excited state MD, and nonadiabatic

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MD.

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Theoretical studies for systems with open-shell molecules have attracted much

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attention.37-38 The fragment molecular orbital (FMO) theory based on fragmentation approach

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has been used to perform MD simulations for open-shell systems.39-41 Grimme et al. studied the

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decomposition of ionized molecules by MD simulations. The computed mass spectra extracted

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from these simulations agreed with experimental electron impact mass spectra.42-43

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When a gas-phase polyatomic molecule interacts with the laser field, it can undergo

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multiphoton absorption such that so-called “ladder switching” or “ladder climbing” processes

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take place.44-46 In the former case, the polyatomic molecule first dissociates and neural fragments

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absorb additional photons to ionize, whereas in the latter case, the polyatomic molecule first

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ionizes and the ion absorbs additional photons to dissociate. The two processes are in

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competition. It is difficult to determine whether the “ladder switching” or “ladder climbing”

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dominates.

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In this work, TNM is used as a test model. The photofragmentation dynamics of closed-

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shell neutral TNM and open-shell TNM+ are explored by a novel spin-unrestricted time-

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dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi oscillations

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and principles similar to trajectory surface hopping, with a mid-intensity field approximation. A



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clear understanding of the photodissociation of this small compound is of critical importance to

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the elucidation of the photochemical reactions involved in the case of more complex molecules. The procedure of spin-restricted TDESMD is described in detail in previous work.34-36, 47-

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48

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(β) components. As an approximation, the spin flip transitions are not considered, assuming

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vanishing spin-orbit coupling. In the framework of u-TDESMD calculations, an ensemble

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average provides results equivalent to a single trajectory, as rationalized below. Therefore, one

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uses a single trajectory instead of an ensemble average. This feature of the methodology

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originates from zero initial velocities of all ions at the initial time as an approximation. Even if

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one waives this zero-velocity approximation, the amount of kinetic energy transferred to the

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system from light does exceed initial kinetic energy of ions = 0 ≪ ≫ 0. Thus, the

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dependence on sampling of initial conditions is expected to vanish. In the following equations,

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the label = α or β is used to indicate spin. The Kohn-Sham orbitals , , , orbital

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energies , , and total density of electrons can be obtained from the geometry

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optimized model at initial time and from any updated geometry at later times according to

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density functional theory (DFT) procedures.49 The density matrix elements , serve as

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weight coefficients for the total density of electrons

Here we use the spin-unrestricted method i.e. u-TDESMD to isolate spin up (α) and spin down

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∗ , = ∑ , , , , .

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The time evolution of electronic degrees of freedom can be calculated by solving the

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equation of motion for the density operator ρ!" cast in terms of the Liouville−von Neumann

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superoperator ℒ" and Redfield superoperator ℛ" ,

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&

ρ! = ℏ )F+" , ρ!" , + . &' "

/,01 /

2

/033

= −ℒ" + ℛ" ρ!" .


(1)

(2)

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: ∙ , where 5+ represents ground state Here Fock matrix reads F+" = 5+ + 6+ 78 − 9

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: transition dipole operator, = 0 ∙ Fock matrix, 6+ 78 nonadiabatic couplings, 9

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cos? @ABC electric field with ? @ABC /2F the laser field frequency.50-51 Application of rotating

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wave approximation (RWA) in the interaction picture allows numerical propagation of equation

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of motion using slowly changing time−dependent Hamiltonian Fock operators for exploring

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coupled electronic and nuclear trajectory of long duration with acceptable precision.52 In this

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∙ case, the light-to-matter interaction operator becomes time−independent, according to [9

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,I ∙ 0 . The electrons transitions 0 → 1 and 0 ← 1 are induced with the ] ,I ≈ 9

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M@N M@N ,I ∙ 0, transition−specific Rabi frequency ? ,I /2F upon laser perturbation, ℏ? ,I =9

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,I is the electric−dipole−moment matrix element in the independent orbital where 9

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approximation (IOA).53 Propagation of electronic degrees of freedom (Eq. 2) is modeled as an

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approximation of optically driven Rabi oscillations with instantaneous and discrete hops in the

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elements of density matrix as an educated guess for solution. During a transition event, a

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stepwise change in occupations of two participating orbitals 0 and 1 develops in time as then

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introduced by, ∆Q

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, = 1 → , = 0, ∆ , = −1

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,II = 0 → ,II = 1, ∆ ,II = +1.

(3a)

∆Q

RST,UV

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Electronic dissipative transitions .

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78 are computed along nuclear molecular dynamics trajectory. Nonadiabatic couplings 6 ,I

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trajectory

RQ

according

2

(3b)

RAA

are facilitated by nuclear motions computed along

to

the

on−the−fly


procedure

as,


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ℏ

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∗ ∗ 78 = − X / [ , 6 ,I , ,I , + ∆ − ,I , , , + W∆Q

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∆].54-56 The autocorrelation function 9 ,IY Z is processed by averaging along the duration

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of the trajectory as, 9 ,IY Z = \ X] 6 ,I + Z6 ,Y / . A Fourier transform of the

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autocorrelation function provides elements of Redfield tensor, which control the dissipative

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dynamics of the density matrix, .

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78 ∙ _. calculations, the nonadiabatic coupling is negligible as 6 ,I < _9

[

\

RST,UV RQ

2

RAA

= ∑ , ,IY , . Note that in u-TDESMD

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The updated total density + ∆ is then used in the Kohn−Sham self−consistent

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procedure, and determines the energy gradient and forces 5 [] imposed on each nuclei of

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the model, R`

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RQ `

= 5 []/9 .

(4)

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Coupled electronic and nuclear degrees of freedom are propagated forward in time using Eq.1

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for feedback.

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In u-TDESMD algorithm, the leading order process is represented by the molecule

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undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic

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energy is accumulated to overcome the dissociation barrier. Time of elementary reaction event is

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determined by observing bond distance over time / and comparing it with a threshold value

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/′. In the case of bond breaking or bond formation events at the instant of time ′, |/′ − /′| >

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, where is the tolerance.57-59 The u-TDESMD trajectory generates “hot” intermediates with

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nonzero kinetic energy. It is difficult to calculate the activation energy of dissociation reaction

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because of uncertainty in total energies of “hot” intermediates. Therefore, an attempt to identify

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high precision activation energy has been committed based on artificial removal of kinetic 6 ACS Paragon Plus Environment

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energy component. One uses a post-processing technique referred to as “cooling” to extract the

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intermediates with no influence of kinetic energy from the trajectory. A single point energy

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calculation is applied to “hot” intermediates. Thus, the kinetic energy does not contribute to the

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barrier height. It should be noted that the post-processing treatment does not affect u-TDESMD

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simulations, because the “cool” intermediates don’t enter into the trajectory. Along the u-

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TDESMD trajectory of coordinates and momenta, the energy and forces are evaluated at each

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QhQ = time step as a function of both nuclear positions d e and momenta df e as ghQ

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QhQ [d e, df e] . However, the “cooling” concept is based on artificial resetting of

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QhQ = QhQ [d e, df ≡ 0e]. momenta to zero value as ihh

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The u-TDESMD algorithm describes the dissociation of polyatomic molecules or ions

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under intermediate laser field. Photoionization mass spectra can be extracted from u-TDESMD

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trajectories with certain approximations. (i) Intensities of features in the mass spectra are

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determined by the number of corresponding fragments in the trajectory. (ii) A post-dissociation

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ionization process, as a sampling method, is assumed rather than modeled for all fragments. Each

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fragment is treated with a single positive charge. (iii) The distribution of features in the mass

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spectra is determined by a finite−width Lorentzian function, where the center is located at the

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molecule weight of corresponding fragments and the width is set as 0.1.

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In the simulation cell, a 9 Å vacuum in each direction, x, y, z, is added to minimize

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interactions between the fragments and penetrations into the simulation cell, mimicking low

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pressure gas or vacuum environment. Calculations are done in a basis set of Kohn−Sham (KS)

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orbitals computed in DFT using the Perdew−Burke−Ernzerhof (PBE) form of generalized

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gradient approximation (GGA) with the projected augmented wave (PAW) potentials as

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implemented in the Vienna Ab initio Simulation Package (VASP).60-64 Starting from the 7 ACS Paragon Plus Environment


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optimized geometry, u-TDESMD calculations were performed for 600 fs using a time step of 1

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fs with an inverse Rabi frequency of 10 fs. Atomics models were visualized using VESTA

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software.65

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Figure 1 shows the basic electronic structure of neutral TNM and TNM+ computed by

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PBE functional. For the neutral TNM the density of states (DOS) (Figure 1a) and calculated

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absorption spectra (Figure 1b) of spin α and spin β components are identical, since it is in

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closed-shell system with spin paired electrons. The calculated band gaps are k,l = k,m = 3.23

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eV. In contrast, the DOS (Figure 1c) and calculated absorption spectra (Figure 1d) of TNM+ are

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different for spin α and spin β components. The calculated band gaps are k,l = 3.28 and

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k,m = 0.20 eV.

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There are several features in the calculated absorption spectra. The electronic transitions

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with the leading contribution to these features are summarized in Table 1 and are explored in u-

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TDESMD to simulate the photodissociation reaction. In the case of neutral TNM, we only

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consider the spin α component to save computational resources. One would expect that

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trajectories induced by spin α and spin β transitions are identical because of indistinguishable

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electronic structures. In the case of open-shell TNM+, both spin α and spin β components are

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considered. In addition, there are several features in the range between 0 and 2 eV for the

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calculated absorption spectrum of TNM+ of spin β component. These features might contribute

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to the photofragmentation of the open-shell system. However, they are not explored in u-

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TDESMD simulations to make a consistent comparison study in terms of UV-Vis range of

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excitation energy.


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Figure 2 shows the energy diagram of intermediates in u-TDESMD trajectory with

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electrons hopping between the orbital pair (HO–7, LU+3)α by using neutral TNM molecule as

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the starting point. Note the calculated absorption spectrum changes during the trajectory such

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that the most probable transition will involve different orbital pairs. At the moment, the orbital

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pair [i(0), j(0)] is chosen based on the calculated absorption spectrum of the geometry optimized

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initial reactant. In future, we plan to use time-dependent orbital pairs [i(t), j(t)] for u-TDESMD

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calculations.

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In Figure 2, the initial reactant and final product are geometry optimized. The single

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point energy calculation is performed for “hot” intermediates generated in the trajectory to get

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the total energy without kinetic energy of bond contraction and bond elongation. The first 500 fs

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of the trajectory is characterized by the sequential elimination of the NO2 group. Bielski and

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Timmons studied the photolysis of TNM at 77 K and found the primary pathway is a split into

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NO2 and C(NO2)3.3 In our simulation, the ejection of NO2 is observed at 131 fs. The ejection of

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another NO2 is observed at 216 fs. It should be noted that in this study dissociated fragments are

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kept inside the simulation cell instead of being removed. Thus, the ejected fragments undergo

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recombination and re-elimination in the subsequent trajectory. One can practically overcome this

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“trapping artifact” by allowing the simulation cell to expand during the trajectory. Later on, a

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local maximum of the total energy is found at 540 fs where ejected NO2 groups drift away from

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the remaining C(NO2)2 fragment, as evidenced by the increase of C-N distance in the diagram.

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The activation energy of Ea = 8.72 eV is obtained by subtraction of the total energy of

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intermediate at 540 fs from the total energy of the reactant i.e. geometry optimized TNM

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molecule. Subsequently, one observes the isomerization of C(NO2) fragment, if we take the other

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bound NO2 group as a non-participating spectator. The isomerization reaction is essential for the 9 ACS Paragon Plus Environment


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elimination of NO fragment. Other studies on the decomposition of the nitro-functionalized

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compounds using different approaches also show the isomerization C(NO2) → C-O-N-O.12, 66-67

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Finally, the spectator NO2 group is eliminated giving rise to CO, evidenced by the decrease of C-

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O distance in the diagram. The product is composed of CO, NO, and 3 NO2 gases. The total

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energy difference between the final product and the initial reactant is about ∆E = 1.08 eV. In this

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work, the u-TDESMD simulations were performed for a short period of time. In such cases, the

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system probably doesn’t accumulate enough kinetic energy to overcome the dissociation barrier

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to reach the global minima. However, the final product from the simulation agrees with

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experimental results.8 Bock and Zanathy studied the pyrolysis of gas-phase TNM and found the

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compound decomposed completely into gaseous CO, NO, and 3 NO2 components over 580 K.8

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The dissociation reactions in Figure 2 can be summarized in equations (5a) – (5d), gu

CNOW t vw CNOW x + NOW , ∆E = 2.66 eV

208 209 210 211

gu

CNOW x + NOW vw CNOW W + 2 NOW , ∆E = 1.97 eV gu

CNOW W + 2 NOW vw OCNOW + NO + 2 NOW , ∆E = 0.38 eV , E@ = 4.09 eV gu

OCNOW + NO + 2 NOW vw CO + NO + 3 NOW , ∆E = −3.93 eV

(5a) (5b) (5c) (5d)

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where ∆E is the total energy difference between the product and reactant for each reaction and Ea

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is the activation energy.

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The energy diagram of intermediates in u-TDESMD trajectory with electrons hopping

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between the orbital pair (HO–2, LU+6)α and (HO–6, LU+4)β by using TNM+ as the starting point

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can be found in Figure S1 and S2 in the supporting information, respectively. In both cases, the

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major dissociation pathway is still the elimination of NO2 group. The isomerization of C(NO2) is

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again observed and leads to the formation of CO by the elimination of NO. It is found that the


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cation is much easier to dissociate than the neutral species. In Figure S1, the activation energy

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(Ea), energy difference between final product and initial reactant (∆E), and the time scale of the

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dissociation reaction are about 3.35 eV, -1.96 eV, and 300 fs, respectively. In Figure S2, the

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corresponding values are about 4.65 eV, -2.04 eV, and 500 fs, respectively. Later increases in

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energy can be interpreted as an artifact of the finite cell size, and collisions of the products. The

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CO, NO, and NO2 yield of each trajectory can be found in Table 1.

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We also calculate the total energies of reactant, transition state i.e. intermediate with the

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highest total energy, and product of Figure 2, S1 and S2 using DFT method with

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Heyd−Scuseria−Ernzerhof (HSE06)68-69 functional implemented in VASP and time-dependent

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DFT (TDDFT)70 method with Becke, 3-parameter, Lee-Yang-Parr (B3LYP)71 functional

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implemented in Gaussian72, see Figure 3. One observes that Ea and ∆E vary with different

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functionals. Figure 3 suggests the dissociation reaction starting with the cation is exothermic,

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whereas the one starting with the neutral is endothermic.

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Figure 4a-4c show simulated mass spectra based on u-TDESMD trajectories. As a

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reference, the experimental electron ionization (EI) mass spectrum73 is shown in Figure 4d. The

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dominant feature is NO2+ for all mass spectra. Features such as C(NO2)3+ and C(NO2)2+ are

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present in both simulated and experiment mass spectra, even though the relative intensities are

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different. Some distinct features such as CO+ are only found in the simulated mass spectra, while

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CN+ and CNO+ are only observed in the experiment EI mass spectrum. It should be pointed out

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that the feature corresponding to the molecular ion is strong in the simulated mass spectrum

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based on the simulation starting with neutral TNM. However, it is weak in the simulated mass

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spectra based on simulations starting with TNM+.

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fragment than the neutral, as the ionization makes the bond length longer and thus weakening the

It confirms that the cation is easier to



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bonding. The molecular ion is not observed in the experimental EI mass spectrum. This is not

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surprising considering the high-energy electron impact creates high-energy molecular ion that

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subsequently dissociates.

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In summary, a recently developed u-TDESMD algorithm taking into account spin

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configurations has been used to model the photodissociation of TNM. Both the closed-shell

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neutral TNM and open-shell TNM+ have been used as starting points for simulations. It is found

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that the cation is easier to dissociate than the neutral species in terms of shorter reaction time and

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lower activation energy. The major dissociation pathway is the ejection of NO2. The final

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products in certain trajectories are completely composed of gas-phase CO, NO, and 3 NO2

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components. These observations agree well with experimental findings.3,

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present simulations show that the isomerization of C(NO2) fragment is a necessary step for the

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formation of CO and NO. The ab initio mass spectra are extracted from u-TDESMD trajectories

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with certain approximations. The interpretation of computational results provides insights about

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reaction mechanisms and product distributions that are not available in photofragmentation

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experiments. The u-TDESMD methodology has the potential to model and monitor

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photoreactions with open-shell intermediates and radicals.

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Corresponding Author

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*(D.S.K.) E−mail: [email protected]

8

In addition, the

260 261

Notes

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The authors declare no competing financial interest.

263 264

Acknowledgments

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This research was supported by NSF award EPS−0903804, CHE−1413614 for methods development and

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by DOE, BES −Chemical Sciences, NERSC Contract No. DE−AC02−05CH11231, allocation Award


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89959 “Computational Modeling of Photocatalysis and Photoinduced Charge Transfer Dynamics on

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Surfaces”. DSK acknowledges support from NDSU Department of Chemistry and Biochemistry and

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College of Science and Mathematics. BR gratefully acknowledges support from the North Dakota State

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University Center for Computationally Assisted Science and Technology and the DOE Grant No.

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DE−SC0001717, as well as support from NSF under ND EPSCoR Award #IIA-1355466 and by the State

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of North Dakota.

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Supporting Information Available: Energy diagrams of intermediates in u-TDESMD trajectory by

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using TNM+ as the starting point.



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References

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449 450



(a)

(b) B C

D E F

Absorption (a.u.)

A

Density of states

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

(c)

(d)

d

f g

e a b c a' b'

e'

c'

f' g'

d' -2

0

2

4

6

8

0

2

4

h' 6

8

Transition energy, eV

Orbital energy, eV 451 452

Figure 1. (a, c) Spin-polarized density of states for TNM and TNM+, respectively. The filled area

453

represents occupied states whereas the unfilled area represents unoccupied states. (b, d) Calculated

454

absorption spectra (dashes) for TNM and TNM+, respectively. The solid vertical sticks indicate the

455

electronic transitions of the most contribution with the height equal to the oscillator strength. In each

456

panel the upper curve indicates spin α component and the lower indicates spin β component.

457


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


458 459

Figure 2. Energy diagram of intermediates extracted from u-TDESMD simulation with electrons hopping

460

between the orbital pair (HO–7, LU+3)α by using TNM as the starting point. The initial reactant and final

461

product are geometry optimized. The single point energy calculation is performed for intermediate atomic

462

models to get the total energy without kinetic energy of bond contraction and bond elongation. The red

463

solid, blue short dashes, green long dashes represent total energy, average C-N distance, and C-O distance

464

for selected atoms. This diagram illustrates activation energies for several steps of ultrafast cracking

465

reactions.

466



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

467 468

Figure 3. Energy diagram of three important points extracted from Figure 2, S1, and S2 and computed by

469

different functional upon single point energy calculations. In each case, the left and right lines indicate the

470

initial reactant and final product, respectively. The middle line represents the intermediate with the

471

highest total energy. Total energies are shown with respect to the total energy of reactant for each curve.

472

The red, blue, and green codes indicate results at the level of DFT/PBE, DFT/HSE06, and

473

TDDFT/B3LYP, respectively.

474 475


N+ O+

C(NO2 )4+

C(NO2 )3+

CNO 2+ / OCNO +

(b)

C(NO2 )2 +

CO + NO+

OCNO 2+

(a)

Intensity (a.u.)

(c)

0

476

CNO +

(d) CN+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NO2 +

Page 21 of 22

50

100

150

200

m/z

477

Figure 4. Simulated mass spectra from u-TDESMD trajectories with electrons hopping between (a)

478

orbitals pair (HO–7, LU+3)α by using TNM as the starting point, (b) orbitals pair (HO–2, LU+6)α, and (c)

479

orbitals pair (HO–6, LU+4)β by using TNM+ as the starting point. (d) The experimental electron

480

ionization mass spectrum of TNM.

481



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

482 483

Page 22 of 22

Table 1. Electronic transitions contributing to u-TDESMD simulations and theoretical yields of photofragments feature (Figure 1) A B C D E F a b c d e f g a' b' c' d' e' f' g' h'

transition energy (eV) 3.39 4.52 4.94 7.52 8.15 8.50 3.43 3.90 4.35 4.62 4.99 7.98 8.45 3.23 3.75 4.31 4.54 6.20 7.72 7.44 8.23

oscillator strength 0.019 0.077 0.045 0.067 0.067 0.065 0.023 0.018 0.045 0.090 0.048 0.082 0.064 0.025 0.022 0.045 0.135 0.057 0.100 0.065 0.079

orbital pair (HO-3, LU)α (HO-7, LU+3)α (HO-10, LU+2)α (HO-1, LU+4)α (HO-5, LU+4)α (HO-2, LU+7)α (HO-2, LU)α (HO-4, LU)α (HO-9, LU)α (HO-7, LU+3)α (HO-10, LU+1)α (HO-2, LU+4)α (HO-2, LU+6)α (HO-1, LU+1)β (HO-4, LU+1)β (HO-8, LU+1)β (HO-6, LU+4)β (HO-17, LU)β (HO-1, LU+5)β (HO-20, LU)β (HO-4, LU+5)β

484


NO2 0.24 0.12 0.23 0.25 0.03 0.10 0.17 0.11 0.12 0.23 0.28 0.57 0.16 0.14 0.08 0.33 0.01 0.42 0.19

fragment yield NO CO 0.02 0.03 0.03 0.03 0.06 0.24 0.61 0.06 0.21 0.13 0.32 0.06 -

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time

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