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Quantum Chemical Study on UV-vis Spectra of Microhydrated Iodine Dimer Radical Anion A. K. Pathak,† T. Mukherjee,† and D. K. Maity*,‡ Radiation and Photochemistry DiVision and Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai-400085, India ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: October 3, 2009
UV-vis spectral properties of I2•- · nH2O systems are reported based on ab initio quantum chemical calculations. Second order Moller-Plesset perturbation theory is applied for geometry search and time dependent density functional theory is applied to study excited state properties. Relativistic corrections are considered for all the calculations. Geometry search is carried out applying simulated annealing combined with Monte Carlo sampling method. Strong optical absorption band of these hydrated clusters, I2•- · nH2O in the UV-vis region is assigned to σg f σu* type valence electron transition in contrast to charge transfer to solvent spectra in I- · nH2O systems. Simulated optical absorption profile of I2•- · 8H2O system is in excellent agreement with the reported aqueous phase spectrum of iodine dimer radical anion. 1. Introduction Because of the existence of a large number of electronic excited states, molecular iodine (I2) acts as an important system in the field of molecular spectroscopy. The valence orbital occupancy of molecular iodine in its ground state is σg2πg4πu4σu0. When an electron is added to the I2 molecule, it becomes iodine dimer radical anion system (I2•-), an open shell doublet system with the valence orbital occupancy in its ground state as σg2πg4πu4σu1. This radical anion system has been studied extensively by applying photoelectron spectroscopy.1-7 As anions are mass selectable and have weak interaction with nonpolar solvent molecules, small anionic clusters having negatively charged chromophore have been considered as model systems for studying the influence of solvation on various fundamental molecular properties and processes.8,9 In this regard, iodine dimer radical anions (I2•-) embedded in the Ar, CO2, OCS, and CS2 solvents have been studied extensively by different research groups applying various experimental and theoretical techniques.10-18 Recently, a study based on firstprinciple electronic structure theory has been reported on iodine dimer radical anion (I2•-) embedded in strongly interacting solvent water molecules.19 The structure, stability, and IR spectra of I2•- · nH2O systems have been reported on the basis of density functional theory (DFT) study without any consideration of relativistic effect. It is reported in literature that a strong absorption band is observed in UV region when a number of water molecules are attached to iodine anion forming I- · nH2O clusters and, in bulk aqueous solution, forming I-(aq).20-22 This absorption spectrum originates from the concerted action of solvent water molecules and termed as charge-transfer to solvent (CTTS) spectra. The excited state is a dipole bound state, and the polarity of the solvent molecules plays the key role for the origin and nature of CTTS spectra. In the excited state, the charge is transferred to the solvent molecular orbital energy levels from that of solute. Iodine dimer radical anion (I2•-) may be formed when an iodine atom (I) is attached to iodide anion * Corresponding author. E-mail:
[email protected]. † Radiation and Photochemistry Division. ‡ Theoretical Chemistry Section.
(I-). The I2•- radical anion acts as a specific one electron oxidant species and absorbs strongly in aqueous solution with optical absorption maximum at ∼380 nm.23 However, no theoretical interpretation is available in the literature elucidating the origin of such optical absorption band in I2•- and I2•- · nH2O systems. In this regard, we have studied the I2•- · nH2O cluster systems applying electronic structure theory at second order Moller-Plesset (MP2) level including relativistic corrections. Efforts have been put forth to identify the global minimum energy structures of these hydrated clusters by applying a Monte Carlo based simulated annealing procedure. UV-vis spectra of I2•- · nH2O hydrated clusters have been simulated for these global minimum energy structures (n ) 1-8) and compared with the reported transient optical spectra of iodine dimer radical anion in aqueous solution. Thus, electronic transitions responsible for the optical absorption spectra of I2•-(aq) have also been assigned following time dependent density functional theory (TDDFT) based excited state calculations. Effect of macroscopic hydration on these finite size hydrated clusters is also examined by applying COSMO solvation model. 2. Theoretical Methods Geometry optimizations have been carried out applying second-order Moller-Plesset (MP2) perturbation theory including relativistic corrections adopting triple split valence, 6-311++G(d,p) set of basis function (I is treated with 6-311G(d) basis set).24 The quasi Newton-Raphson based algorithm has been applied to carry out geometry optimization for each of these molecular clusters with various initial structures designed systematically following a bottom-up approach to determine the most stable one. Monte Carlo based simulated annealing approach is adopted to find out the global minimum energy structure of different size molecular clusters applying BHHLYP correlated hybrid density functional.25 Geometry of the predicted equilibrium structures are then further refined applying MP2 theory. Hessian calculations are performed for all of the optimized minimum energy structures to check the nature of the equilibrium geometry. Electron correlated method, namely, time dependent density functional theory (TDDFT) with BHHLYP density functional, is applied to study a few low lying
10.1021/jp9087157 2010 American Chemical Society Published on Web 10/20/2009
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excited states and to generate UV-vis spectra. This hybrid density functional includes 50% Hartree-Fock exchange, 50% Slater exchange, and the additional correlation effects of the LYP functional.26 It is reported in the literature that BHHLYP functional performs well to describe such hemi-bonded open shell doublet systems.19,27 Moreover, spin-orbit interaction is expected to be significant in these iodine containing hydrated clusters. Thus, geometry search has been carried out including spin-orbit interaction applying a relativistic scheme by eliminating small components which provide both scalar (spin free) and vector (spin-dependent) relativistic corrections.28 In recent years, various models are proposed for long-range corrections in DFT functionals replacing the DFT exchange part with the HF exchange to predict accurate ground state geometrical parameters and accurate vertical excitation energies within timedependent density functional formalism.29 At present, density functional PBE0 with long-range correction based on a modified exchange energy density (LRC-µPBE0) because of Hirao and co-workers29a is applied to calculate ground state geometry and excited state energy parameters. The value of Coulomb attenuation parameter, µ, is taken as 0.33 in all calculations. This DFT functional with long-range correction, LRC-µPBE0, has been shown to predict accurate vertical detachment energies of water clusters with an excess electron, and absorption and fluorescence properties of oligothiophene biomarkers.30 Note that the two hybrid functionals, BHHLYP and LRC-µPBE0, predict ground state geometry very close to the same at MP2 level of theory in these hydrated clusters. Effect of macroscopic hydration is also tested by searching ground state minimum energy structures of these hydrated clusters applying COSMO model with BHHLYP functional. Excited state calculations are also carried out on these solvent modified geometries applying TDDFT with BHHLYP functional. Simulation of absorption profiles is carried from a few low lying excited states with Gaussian line shape; that is,
ε(E) ) A
∑ i
(
(E - Ei)2 fi exp B ∆i ∆i2
)
where, , E, f, and ∆ are molar extinction coefficient, transition energies, oscillator strength, and half bandwidth, respectively. All electronic structure calculations are carried out applying the GAMESS suite of the ab initio program system on a LINUX cluster platform.31 Visualization of molecular geometry and molecular orbitals (MOs) is carried out by the MOLDEN program.32 Basis sets 6-311G(d) for I are obtained from the Extensible Computational Chemistry Environment Basis Set Database, Pacific Northwest National Laboratory. 3. Results and Discussion Geometry optimization is carried out at MP2/6-311++G(d,p) including relativistic correction on various possible initial structures of I2•- · nH2O clusters (n ) 1-8). A number of close lying minimum energy structures are obtained for each size of cluster. A Monte Carlo based simulated annealing procedure is then applied to find out the global minimum energy structure of each size of the molecular clusters. The global minimum energy structure that is predicted for each size hydrated cluster, I2•- · nH2O, is reoptimized at MP2 level, and the final structures are displayed in Figure 1. It is clear from Figure 1 that the I2•- · nH2O clusters are stabilized by double hydrogen bonding (DHB), single hydrogen bonding (SHB), and inter-water
Figure 1. Most stable structure at MP2/6-311++G(d,p) level of theory including relativistic correction for (I is treated by 6-311G(d) set basis function) (i) I2•- · H2O, (ii) I2•- · 2H2O, (iii) I2•- · 3H2O, (iv) I2•- · 4H2O, (v) I2•- · 5H2O, (vi) I2•- · 6H2O, (vii) I2•- · 7H2O, and (viii) I2•- · 8H2O clusters. I atoms are shown by the largest pink color spheres; the smallest spheres refer to H atoms, and the rest (red in color) correspond to O atoms in each structure.
hydrogen bonding (WHB). It is noted that conformers having H-bonded water network (WHB) are more stable over the other structures where H2O units are connected to the anion moiety independently either by SHB or DHB for a particular size of cluster. A H-bonded water network having two, three, or four H2O units is present in the different conformers of these clusters. The initial structures for global minimum energy search of these hydrated clusters are based on previous reported structures on similar systems.19,33 A hydrated cluster having cyclic water network units is the most stable conformer for each size cluster. I2•- · nH2O clusters of size n ) 4-8 contain at least one fourmember water ring (see Figure 1). In each case, the distance between the two I atoms is ∼3.3 Å; the distance between I and H-bonded H atoms is 2.9-3.5 Å (SHB and DHB bond), and the distance between O and H atoms in the inter-water network is 1.8-2.1 Å (WHB bond). Time dependent density functional theory with BHHLYP density functional is applied to find out a few low lying excited states for all hydrated clusters and for the isolated dimer radical anion (I2•-). To observe the effect of cluster size (number of solvent H2O molecules attached) and hydration on the excited state electronic properties (optical absorption wavelength, λ) of these hydrated clusters, excited state properties of iodine dimer radical anion (I2•-) are calculated with the successive addition of solvent water molecules. Relativistic correction is also considered for calculation of excited states because of the presence of a heavy atom like I. It is observed that BHHLYP and PBE0 with long-range correction (LRC-µPBE0) hybrid functionals produce best ground state geometry with respect to MP2 level of theory. So both the hybrid functionals, BHHLYP and LRC-µPBE0, are applied to study the excited states of these hydrated clusters. The optical absorption maxima (λmax) for the isolated dimer radical anion (I2•-) is predicted as 408 nm at TDDFT-BHHLYP/6-311++G(d,p) level of theory considering BHHLYP/6-311++G(d,p) geometry. However, if macroscopic solvation is included in geometry optimization adopting COSMO model, calculated λmax at TDDFT-BHHLYP level gets blue shifted by 6 nm. The λmax values of different finite size hydrated cluster, I2•- · nH2O, are also calculated at the same level of theory and listed in Table 1. Excited state calculations suggest that I2•- · nH2O clusters have strong optical absorption bands in UV-vis region. λmax values of these hydrated clusters get blue
Microhydrated Iodine Dimer Radical Anion
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TABLE 1: Calculated Optical Absorption Maxima (λmax) of I2•- · nH2O Clusters in Nanometers at TD-DFT Level with BHHLYP and Long Range Corrected PBE0 (LRC-µPBE0) Density Functionals with and without Relativistic Correctiona TDDFT (LRC-µPBE0)//MP2c
TD-DFT (BHHLYP)// BHHLYP hydrated species
without relativistic correction
with relativistic correctionb
TD-DFT (BHHLYP) //MP2c
I2•-.H2O I2•-.2H2O I2•-.3H2O I2•-.4H2O I2•-.5H2O I2•-.6H2O I2•-.7H2O I2•-.8H2O
414 415 412 410 413 419 414 409
409, 405 411, 401 405, 401 405, 403 408, 404 408, 402 409, 399 403, 403
392 (0.48) 392 (0.46) 387 (0.47) 387 (0.46) 390 (0.43) 390 (0.42) 392 (0.40) 386 (0.41)
without relativistic correction
with relativistic correction
393 (0.50) 394 (0.48) 388 (0.48) 388 (0.47) 390 (0.44)
386 (0.49) 387 (0.47) 382 (0.47) 382 (0.47) 384 (0.44) 385 (0.42) 387 (0.45) 382 (0.41)
a The basis set applied is 6-311G(d) for I and 6-311++G(d,p) for other atoms. The minimum energy structures are obtained applying either BHHLYP or MP2 method with the same set of basis functions including relativistic correction. Optical absorption maxima are calculated from the vertical excitations of a single electron. b Values in italic numerals are calculated for I2•- system including macroscopic solvation model, COSMO for geometry optimization followed by TDDFT for excited state calculations. c Values in the parentheses are the calculated oscillator strengths.
Figure 2. Contour plot of (A) the highest occupied molecular orbital (HOMO) and (B) HOMO-3 showing they are of σg and σu* type character respectively. The cutoff for all contour plots is taken as 0.05 au.
shifted upon addition of successive solvent water molecules compared with that of the isolated bare I2•- solute. Note that, in the case of I- · nH2O clusters, the strong optical absorption bands in the UV region are due to charge-transfer to solvent (CTTS). In contrary, the strong optical absorption in UV-vis region in I2•- · nH2O clusters is due to valence σg f σu* electronic transition, that is, the transition from HOMO-3 to HOMO. Contour plots of HOMO (σu*) and HOMO-3 (σg) is depicted in Figure 2. Simulated optical absorption spectra of the isolated solute, I2•-, and the microhydrated cluster, I2•- · 8H2O, based on 10 low lying excited states at TDDFT(LRC-µPBE0)/6-311++G(d,p) (I is treated with 6-311G(d) basis set) level of theory including relativistic correction are shown in Figure 3a,b. Gaussian line shaping with fwhm of 100 nm has been considered for these spectral simulations. In the case of the I2•- · 8H2O cluster, the calculated absorption maximum gets blue shifted by ∼26 nm on microhydration in contrast to the calculated 6 nm blue shift observed by applying the COSMO macroscopic solvation model. Note that in both cases excited state calculations are carried out at TD-DFT(LRCµPBE0)/6-311++G(d,p) level of theory. Pulse radiolysis based experimental UV-vis spectrum of I2•- in aqueous solution with λmax ∼ 380 nm is displayed in Figure 3c. It is clear from the figure that the simulated optical absorption profile of finite size hydrated cluster I2•- · 8H2O is in excellent agreement with the measured spectrum in bulk aqueous solution. In short, it is observed that relativistic correction is necessary for both geometry search and excited state study in order to reproduce the excited state absorption profiles in hydrated clusters of the iodine dimer radical anion. We have also noticed that a geometry search at the MP2 level followed by excited state calculations applying TDDFT formalism with LRC-µPBE0 functional is capable of reproducing an experimental optical absorption profile of this hydrated anionic solute. Moreover, unlike hydrated clusters of iodide anion, the origin of the strong
Figure 3. Simulated optical absorption spectra of (a) I2•- at TDDFTBHHLYP/6-311++G(d,p)//COSMO-BHHLYP/6-311++G(d,p) and (b) I2•- · 8H2O at TDDFT- LRC-µPBE0/6-311++G(d,p)//MP2/6311++G(d,p) level of theory (I is treated with 6-311G(d) basis set) based on 10 low lying excited states. Relativistic correction is included in these TDDFT calculations. Gaussian line shaping with fwhm of 100 nm has been considered for spectral simulation. (c) Experimentally measured absorption profile of I2•- in aqueous solution based on pulse radiolysis study.
optical absorption band in the iodine dimer radical anion is due to the σg f σu* type electronic transition. 4. Conclusions UV-vis spectral properties of I2•- · nH2O (n ) 1-8) clusters are studied including relativistic correction. Calculations are carried out applying density functional and ab initio MP2 electronic structure methods. Simulated annealing combined with the Monte Carlo sampling method is applied for the geometry search. Visualization of the appropriate molecular orbital (MO) indicates that the strong optical absorption bands of I2•- hydrated clusters in the UV-vis region is due to σg f σu* electronic transition. Relativistic correction in ground state geometry search and excited state property calculation is important. DFT functional PBE0 with long-range correction (LRC-µPBE0) performs well to study excited state properties. Finally, the simulated absorption profile of the I2•- · 8H2O system is in excellent agreement with the reported aqueous phase spectrum of I2•-. Acknowledgment. Sincere thanks are due to Computer Centre, BARC for providing the ANUPAM parallel computational facility.
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