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Experimental Study and Modeling of the Uv-Vis and Infrared Spectra of the [Vo(o)hheida]- Complex Dissolved in Water 2

Sophia I. Klokishner, Oleg S. Reu, Johannes Noack, Robert Schloegl, and Annette Trunschke J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07128 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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The Journal of Physical Chemistry

Experimental Study and Modeling of the Uv-Vis and Infrared Spectra of the [VO(O2)Hheida]- Complex Dissolved in Water S. Klokishner 1*, O. Reu 1, I. Noack 2, R. Schlögl 2, A. Trunschke 2* 1

Institute of Applied Physics, Academy of Sciences of Moldova, Academy str. 5, MD 2028 Chisinau, Moldova

2

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Corresponding authors: [email protected] (0037322)738604 [email protected]

ABSTRACT: Combined theoretical and experimental studies of the [VO(O2)Hheida]- anion dissolved in water that may serve as a functional model for vanadium haloperoxidase enzymes have been performed. The geometrical structure, absorption and vibrational spectra of this system have been evaluated within the frames of density functional theory (DFT). The obtained theoretical results on the equilibrium structure and optical spectra are in quite good agreement with the experimental data. With the aid of the combination of UV-visible spectroscopy and electronic structure calculations it has been revealed that in the apparent absorption spectra of the [VO(O2)Hheida]- anion the highest in energy band corresponds to a ligand to metal electron excitation, while the band with the maximum at 430 nm arises from the peroxo group. The calculations also reproduce quite well the positions, intensities and the grouping of frequencies in the near infrared (NIR) spectra. The visualization of the calculated vibrations in the energy range of 400-1100 cm-1 has been presented.

1. INTRODUCTION In recent years peroxo vanadium complexes attract considerable interest in different fields of science because of their wide field of applications. Since these complexes represent synthetic structural and functional models for the peroxo form of the vanadium haloperoxidase enzymes and can exhibit insulin mimetic properties or antitumor activity they are very important in biocoordination chemistry.1-6 The other aspect in the studies of peroxo vanadium species is their role in catalytic reactions. In fact they represent intermediates in the catalytic cycle of the vanadium haloperoxidase reactions and also can be used as catalysts for oxygen transfer reactions.7 For a large number of mono- and di-peroxovanadium (V) complexes the determination of the structure by X-ray diffraction showed that the most common geometrical structure for the oxoperoxo complexes is the pentagonal bipyramidal one with the peroxo groups bound in the equatorial plane relative to the oxo ligand.8 The remaining equatorial and apical positions in the coordination shell are occupied ACS Paragon Plus Environment


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by the heteroligands. Various research groups have designed different metal-organic compounds with the intention of mimicking the structure and function of the active peroxidated form. In ref [9] with the aid of Raman and UV-vis spectroscopy it was demonstrated that the vanadia peroxo-oxo umbrella structure is present in vanadium haloperoxidase (VHPOs) enzymes and metal-organic compounds designed to mimic VHPOs. The vast experimental material that is impossible to be cited entirely within the limits of one paper has led to the appearance of a new direction in the study of vanadium peroxo complexes. Density functional theory started to be applied to the examination of catalytic, structural and spectroscopic properties of these complexes. In paper [10] the spectroscopic and structural characterization of a series of four new structurally related high-spin (S = 2) manganese(III)−alkyl peroxo complexes was accompanied by DFT calculations. The ligand environment was shown to subtly vary the extent of the O−O bond activation in these complexes, and theoretical calculations provided an explanation for these observations. The geometric structures of the K[VO(O2)Hheida] complex as well of the molecular ions (VO(O2)Hheida)− and (VO(O)Hheida)− have been evaluated using DFT in paper [11]. The calculated structural parameters of the optimized K[VO(O2)Hheida] and [VO(O2)Hheida]− complexes as well as of the oxygen-deficient [VO(O)Hheida]− ion were compared with the experimental data from X-ray diffraction for crystalline K[VO(O2)Hheida]·2 (H2O).4,5 The comparison shows a close agreement between the optimized structure of this complex and the experimental one. In ref. [11] it was also obtained that the calculated O 1s core ionization potentials vary between the different oxygen species due to their chemical environment and are consistent with results from X-ray photoemission. For the mentioned crystalline complexes the Raman and IR spectra have been also calculated and compared in order to understand the change in the energies and intensities of the vibrations when passing from the K[VO(O2)Hheida] complex to the [VO(O)Hheida]− one. However, a detailed comparison of the observed IR spectra with the experimental one has not been performed. In paper [12] the absorption spectrum of the K[VO(O2)Hheida] complex which mimics crystalline K[VO(O2)Hheida]⋅2(H2O) was examined. The TD-DFT calculations have been performed assuming the coexistence of [VO(O2)Hheida]− and the oxygen deficient [VO(O)Hheida]− complexes. The combined analysis12 revealed that the lowlying absorption ligand-metal charge transfer band, as well as the Raman profile of the K[VO(O2)Hheida] catalyst, are dominated by the actual [VO(O2)Hheida]− complex. At the same time the solvent effects on the shape of the optical and IR spectra of the [VO(O2)Hheida]- complex have not been examined. Therefore, the aim of the present communication is the modeling of the UV-Vis spectra of the [VO(O2)Hheida]− complex dissolved in water and the comparison of the obtained results with the experimental ones. A detailed analysis of the calculated IR spectrum for this anion with the observed one will be also performed. ACS Paragon Plus Environment

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2. EXPERIMENTAL SECTION The procedure of obtaining of the [VO(O2)Hheida]− complex is similar to that described in 4,6. Briefly, potassium vanadate K[VO3] (1.38 g, 10 mmol) was dissolved in 60 ml of destilled water and cooled to 0oC. The ligand N-(2-hydroxyethyl)iminodiactetic acid (H2heida 1.77 g, 10 mmol)) was slowly added under stirring to the initial solution.The resulting yellow solution was further stirred for 20 minutes at 0oC until a clear solution was obtained. Then, aqueous H2O2 (4 ml, 30 %) was added dropwise while the solution turned red indicating the formation of [VO(O2)(Hheida)]−. The pH of the solution was adjusted to 4 and the solution was further stirred for few hours at 5oC. The K[VO(O2)Hheida]·(H2O) complex (sample 17152) was isolated and recrystallized from a water-ethanol mixture. The purity of the sample was evidenced by single-crystal analysis and CHN elemental analysis. The UV/Vis spectra absorption spectra of the K[VO(O2)(Hheida)] complex dissolved in water were recorded in the range between 200 nm and 900 nm in the transmission mode at room temperature with a Perkin Elmer Lambda 25 spectrometer using 1 mm quartz glass cuvettes. In the range of wavelengths 200-600 nm the spectrum of [VO(O2)Hheida]− dissolved in water exhibits two pronounced bands. The broad band in the range of 200-250 nm is usually assigned to the electron transfer from the oxygen orbitals to vanadium 3d orbitals and testifies the presence of the V=O bond. The maximum at ∼430 nm in the absorption spectra originates from the peroxo group.5,9 It is worth noting that the addition of different amounts of KCl to the solution containing the [VO(O2)(heida)]− anion does not change the shape of the absorption spectra and testifies only weak interaction between this anion and potassium. The obtained apparent absorption spectrum also does not show any optical bands characteristic of V4+ ions in octahedral oxygen surrounding which usually fall into the range 440-800 nm (see, for instance13,14 ) and, consequently, the presence of V4+ ions can be excluded. Infrared (IR) spectroscopy by the ATR (attenuated total reflection) method which avoids the problem of strong attenuation of the IR signal in highly absorbing media was applied to the system under examination. The ATR spectra were recorded using a Varian-670 FTIR (Fourier-transform infrared) spectrometer equipped with a nitrogen-cooled MTC detector. An ATR accessory (Gladi ATR, PIKE Technologies) with a diamond internal reflection element was applied.

3. COMPUTATIONAL DETAILS All calculations including the optimization of the structure of the [VO(O2)(Hheida)]− anion dissolved in water as well as the calculation of its absorption and IR spectra have been performed within the frames of the density functional theory (DFT)15 using the ORCA program package.16,17 In our work we have employed the gradient corrected revised Perdew-Burke-Ernzerhof (REVPBE) functional. Representative references for the REVPBE functional one can find in papers [18-24]. In ACS Paragon Plus Environment


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11,25,26

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this functional has been successfully applied for the evaluation of geometric, electronic and

spectroscopic characteristics of a series of molecular complexes containing transition metal ndions in oxidation degrees complying with the unfilled d-shell. In

11

the geometric and electronic

properties of the K[VO(O2)Hheida] complex and molecular ions (VO(O2)Hheida)− and (VO(O)Hheida)− have been evaluated using the REVPBE functional. This functional has been proved to describe quite well oxygen core excitations in bulk MoO3 as well as in different molecular molybdena-silica models.25 The REVPBE functional was also successfully validated in examination of apparent absorption spectra of bulk TiO2 and

different titania, vanadia, and

vanadia-titania clusters located on the surface of mesoporous silica SBA-15.26 The calculations of the absorption and vibrational spectra of the [VO(O2)Hheida]- anion were performed using the polarized valence Ahlrichs-TZVP basis sets27, 28 of triple ς quality for all the atoms in combination with the def2-TZVP/J Coulomb fitting basis for the resolution of identity. Calculations labeled RIJCOSX

29

used the COSX approximation for the exchange terms in

conjunction with the RI-J approximation, in fact this approximation treats the coulomb term via RI and the exchange term via seminumerical integration.16,17 The molecular orbitals of the [VO(O)Hheida]- complex have been quantitatively calculated during the optimization of the structure of the complex by adding the command Print[P_Mos].16,17 The TDDFT method within the ORCA package was applied to calculate the excited state energies, gradients and equilibrium geometries of the examined anion as well as of the IR spectra. It should be underlined that the excitation energies have been computed with the REVPBE optimized geometry of the [VO(O2)(Hheida)]− anion. The ground-state energy and density were converged to 10-8 a.u. and 10-7 a.u., respectively (ORCA keyword TightSCF). The ORCA program also provides the values of adiabatic minima transition energies calculated as differences between the excited and ground-state energies in their equilibrium structures. A conductor like screening model was applied using water as the solvent of choice (COSMO, H2O, = 80.4, = 1.33). It should be underlined that in the recent versions of ORCA the COSMO option 30 that accounts for the influence of the solvent on the

properties of the complex can only be explored in the optimization of the structure of the dissolved complex, however, this option is not implemented for the analytical Hessian. Therefore, the calculation of the vibrational frequencies has been performed with the option “NumHess true” that allows for the solvent effects. Up to now the COSMO option is also not implemented in ORCA for the calculation of the transition energies. To overcome this inaccuracy from the very beginning we explicitly include quite a few water molecules in the quantum chemical calculations and place these molecules in the vicinity of the peroxo O-O bond and other oxygen atoms that enter in the complex keeping in mind that the Coulomb interaction between the oxygen and hydrogen atoms belonging to the vanadia complex and those of water molecules affect the position and shape of the absorption ACS Paragon Plus Environment

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band. During the calculations we determine the optimal number of water molecules that allow to reproduce the position and shape of the observed UV-Vis and IR spectra (see section “Supporting Information”). For identification of the origin of the transitions in the observed spectra of the [VO(O2)(Hheida)]- anion in different wavelength ranges a detailed analysis of the composition of the DFT calculated molecular orbitals contributing to the initial and final states of the optical transitions is performed. To minimize the deviations from the crystallographically determined geometries, bond angles and bond distances the intramolecular van der Waals interactions were accounted as well by using the vdwgrid3 keyword. All calculations were performed with the aid of grid of 5 (Grid5) and Grid6 for the final energy. For the visualization of the vibrations of the [VO(O2)(Hheida)]− anion in the IR range the program Chemcraft 31 was applied. (Chemcraft, Version 1.8, www.chemcraftprog.com)

4.RESULTS In the geometry optimization of the structure of the [VO(O2)(Hheida)]− anion it has been taken into account that the anion is dissolved in water. For this system the initial values of the bond lengths and bond angles have been taken from paper [6]. To fully consider the water effects at all stages of DFT calculations of the absorption spectra the number of water molecules surrounding the [VO(O2)(Hheida)]− anion has been varied from 8 to 22. The calculations apparently showed that the optimal number of water molecules in the nearest surrounding of the [VO(O2)(Hheida)]− anion, which ensures a good agreement between the calculated and the observed spectra, turned out to be 8 (see section “Supporting Information”, Figs S1- S4). The structure of the [VO(O2)(Hheida)]− anion surrounded by 8 water molecules optimized with the aid of the ORCA package16,17 is depicted in Fig. 1.

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O

H

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H

O1

H

H

C O5 C H

O4

H

N

C

O6

H

V

O7 H H O

H H

O3

O

O

C

O H

H

C

H

H

H O2

C

H H

O8

H

H

H

H O

H O

O

H

H

Fig.1. The optimized structure of the [VO(O2)(Hheida)]− anion dissolved in water. In the figure there are also shown 8 additional water molecules included in the model. The selected structure parameters of the optimized [VO(O2)(Hheida)]− anion dissolved in water are given in Table 1. For comparison in the same Table there are listed the experimental values of these parameters obtained from X-ray diffraction for the crystalline [VO(O2)(Hheida)]− and K[VO(O2)Hheida] compounds. 4-6 .

Table 1. Selected distances ( Å ) and angles ( ° ) for the optimized [VO(O2)(Hheida)]− ion dissolved in watera V-O1 V-O2 V-O3 V-O4 V-O6 V-O8 V-N O2-O3 O1-V-O2 O1-V-O3 O1-V-O4 O1-V-O6 O1-V-N O1-V-O8 O2-V-O3

[VO(O2)(Hheida)]1.614 1.867 1.867 2.089 2.066 2.495 2.252 1.420 107.81 106.68 94.01 94.95 93.92 166.70 44.71

Experiment 6 1.599 1.878 1.875 2.020 2.039 2.246 2.164 1.441 105.55 104.08 93.85 93.94 93.28 168.97 45.11


Experiment 4, 5 1.601(1) 1.865(1) 1.864(1) 2.051(1) 2.038(1) 2.236(2) 2.194(2) 1.432(2) 105.19(7) 105.44(7) 93.93(7) 93.93(7) 92.27(7) 167.72(7) 45.17(7)

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a

O3-V-O4 80.90 79.91 O3-V-O6 124.26 124.13 O4-V-O6 149.01 151.72 150.28(6) O8-V-N 73.66 76.43 75.46(6) The calculated structural parameters for the [VO(O2)(Hheida)]− anion dissolved in water are

compared with the experimental data for the crystalline [VO(O2)(Hheida)]− and K[VO(O2)Hheida] compounds. 4-6

From Table 1 it follows that both for the bond lengths and angles quite a good agreement between the values obtained from DFT optimization and the experimental ones achieved. The agreement criterion for the bond lengths estimated as = ∑

4-6

has been )

(

( )

is

equal to 4.4% and 4.3%, when the calculated bond lengths (Table1) are compared with the experimental data from papers [6] and [4,5], respectively. At the same time it should be mentioned that the differences between the calculated and observed interatomic distances V-O8 and V-N are the largest ones. These deviations may be explained by deficiencies of the DFT approach to describe weak binding at large distances. For instance, the V-O8 distance involves a weakly binding hydroxyl oxygen at a relatively large V-O separation where the present DFT approach may be less reliable. As to the parameter = ∑

)

(! !

(! )

which gives a measure of the

difference between the calculated and observed bond angles it takes on the values 1.7% and 1.5% in both above mentioned cases. Thus, the accepted model reasonably reproduces the magnitudes of the structural parameters. At the same time it should be mentioned that in papers [4-6] the bond lengths and angles have been measured for crystalline samples, and the obtained difference between the calculated and observed parameters can be referred to the solvent effects.

4.1. SHAPE OF THE ABSORPTION BAND The calculations performed at the DFT level testify that the molecular orbitals of the [VO(O2)(Hheida)]− anion are orbitally non-degenerate. In this case the formula for the absorption coefficient of the band arising from the singlet-singlet transition should be applied. Following 32-35 the form-function for the absorption band on the s → p singlet-singlet transition of a cluster looks as follows: Fsp (Ω) =

d ps2 2π

∫

∞

−∞

[

]

Exp i (Ω ps − Ω)t + ϕ p (t ) − Γp t − θ p2t 2 dt ,

where


(1)


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ϕ p (t ) = −

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βj 1 cos(ω j t − iβ j / 2 ) 1 , ∆2jp coth + ∑ ∆2jp ∑ βj 2 j 2 2 j sinh 2

(2)

Ω is the frequency of light, the indices s and p correspond to the ground and excited electronic

states of the optical transition, respectively, "#$ is the matrix element of the dipole moment of the

complex on the % → ' transition, the summation over ( is performed over the discrete spectra of all ground-state vibrational frequencies ω j of the [VO(O2)(Hheida)]− complex dissolved in water,

βj =

hω j kT

, hΩ ps = J p − J s , J i = i V j i − ∑ j

hω j 2

2

q ji ( (i = p, s) , Ji is the adiabatic potential

minimum in the electronic state i, V j is the operator of interaction of the electrons of the complex with the vibrational mode )* of the complex corresponding to the frequency ω j , and, finally,

q ji = − i V j i / hω j is the equilibrium coordinate of the vibrational subsystem in the electronic

state i , ∆ jp = q jp − q js . Since the frequencies of the cluster vibrations form a discrete spectrum, to avoid divergences in the form-function Fsp (Ω) the factor Exp[−(Γp t + θ p2t 2 )] is introduced phenomenologically in the integrand of Eq. (1), here Γp and θ p are the homogeneous and inhomogeneous line width parameters on the transition s → p . Further on the ensemble effects in Eq.(1) are accounted for by assuming a Gaussian shape for distributions in the adiabatic minima energy separations ( θ p is the standard deviation for adiabatic minima separation Ω ps ). When the ground state of the complex is well isolated from the excited ones (i.e. the energy gaps between the

minima of the adiabatic potentials corresponding to the ground % and excited p states exceed

significantly the thermal energy) the absorption coefficient for the whole complex represents a sum of coefficients corresponding to individual % → ' transitions and can be expressed as

+ (Ω) = Ω ∑# -$# (Ω) In the ORCA package

(3) 16, 17, 35

the calculation of the shape of the absorption band of a complex is

based on Eqs (1)-(3). It should be mentioned that for the calculation of the energies of excited states the TDDFT method 36 is incorporated into the ORCA package. In fact within the frames of DFT 15 the ORCA program microscopically evaluates all parameters that enter in Eqs (1)-(3) besides the parameters Γp and θ p which determine the Gaussian or the Lorentz shape of the individual band on

the % → ' transition and are introduced in the program phenomenologically to avoid divergence in

the form-function -$# (Ω) and, correspondingly, in the absorption coefficient +(Ω). However, as it ACS Paragon Plus Environment

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was demonstrated many years ago in the pioneer paper of M.A. Krivoglaz and S.I. Pekar

37

the

integration over . in (1) can be analytically performed in the case of “strong heat release”

0 ≫ 1) when the function ϕ p (t ) is expanded up to terms of second order in t: (∑* ∆*#

2# (.) ≅ 2# (0) + 2#5 (0). + 0 2#55 (0). 0 .

(4)

Then from Eq.(2) it is easily obtained that 0 0 55 2# (0) = 0, 2#5 (0) = ∑* ∆*# 6* , 2# = − ∑* 6*0 ∆*# 89.ℎ

0

0

βj 2

= − σ0# ,

(5)

The presentation of 2# (.) in form (4) can be justified since the main contribution to the

integral over . in Eq.(1) comes from the range of short times.37 Substituting (4) into (1) and

[

]

omitting the factor Exp − (Γp t + θ p2t 2 ) after elementary integration one obtains the form-function

for the absorption band on the singlet-singlet transition s → p of a cluster in the following form

-$# (Ω) =

0?@

AB' C−

(DE = D)

0@

F

(6)

In such a way in the examined case of strong heat release the absorption band on the % →

' transition is described by a Gaussian curve with a maximum at 0 ΩG #$ = Ω#$ + 0 ∑* 6* Δ*#

(7)

Ω = 2>2J0# K 2

(8)

and a half –width equal to

Thus, when the ground state of the complex is well isolated from the excited ones and the condition of “strong heat release” holds the absorption coefficient of the complex looks as follows

+ (Ω ) = Ω ∑ #

0?@

AB' C−

(DE = D)

0@

F

(9)

wherein the summation over the excited states is taken into account. It should be also noted that the case of “strong heat release” can be described in the semiclassical approximation within which the motion of ions is treated in a classic manner. In fact Eq.(9) can be obtained using the expressions of the energies in the adiabatic approximation. The absorption band of the [VO(O2)(Hheida)]− anion dissolved in water has been calculated in two different ways: (a) first the optimization of the structure of the [VO(O2)(Hheida)]− anion dissolved in water has been performed with the aid of the REVPBE functional,18-24 then the Hessian calculations gave the possibility to obtain the displacements ∆ jp and the frequencies ω j . At the next stage the program orca_asa

17

has been applied to the calculation of the transition energies

0 . Finally, assuming for individual % → ' transitions a Gaussian hΩ ps and oscillator strengths "#$

shape with a definite θ p value the absorption coefficient as a function of the light frequency Ω has



been calculated; (b) in the second approach the absorption coefficient was obtained with the aid of

0 Eq.(9), the parameters ΩG #$ , σ0# and "#$ that determine the position, width and intensity of the

absorption band have been taken from the DFT calculations as well. Here it should be mentioned that the curve completely calculated with the aid of the ORCA package (Eq.(1)) broadens with increase of θ p . However, at values of θ p higher than 1000 cm-1 the shape of the curve does not change. At the same time Fig. 2 clearly shows that for θ p =1000 cm-1 the absorption curve calculated following all steps of the ORCA program and that obtained in the approximation of “strong heat release” (Eq.(9)) coincide. This means that the latter approximation i.e. Eq.(9) is valid

1,0

Absorption, a.u.

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0,8 0,6 0,4 0,2 0,0 200

300

400

λ, nm

500

600

700

Fig.2.Apparent absorption spectrum of the [VO(O2)(Hheida)]- complex dissolved in water: black line – experimental data; green line illustrates two coinciding spectra calculated with the aid of Eqs (1) for θ p =1000 cm-1 and in the approximation of “ strong heat release” (Eq.(9) ); red line represents the spectra calculated in the case of “strong heat release”by increasing in 1.06 times the

displacement parameters Δ*# corresponding to the vibrational modes 582 and 591 cm-1.

for the description of the apparent absorption spectra of the [VO(O2)(Hheida)]− anion for θ p ≥1000 cm-1. At the same time in both approaches the calculated position of the maximum of the peroxo band is slightly “red” shifted as compared with the experimental one. The analysis of the obtained numerical values of the parameters ∆ jp , ω j and Ω ps allows to arrive at the conclusion that the calculated “red” shifted position of the band in the visible range arises from the underestimation of the constants Δ*# =

LM$NOP N$QM#NOP N#QR ℏTP

corresponding to the vibrational modes with the frequencies

582 and 591 cm-1 (see description of the infrared spectra in section 4.2). The agreement between the calculated and observed position of the band of the [VO(O2)(Hheida)]− anion in the visible range ACS Paragon Plus Environment

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becomes greatly improved if the values Δ*# for the mentioned vibrations are only increased in 1.06 times. This fact is clearly illustrated by the red curve in Fig.2. It should be also mentioned that the

analysis of the composition of the molecular orbitals participating in the optical transitions allows to make the assignment of the peaks observed in the apparent absorption spectra of the [VO(O2)(Hheida)]− complex dissolved in water. At the same time the full scheme of the molecular orbitals is not presented here since the total number of molecular orbitals of the complex under examination is 1126. In Table 2 there are given the calculated energies of the main transitions contributing to the spectra. All these transitions are spin allowed. In the

Table 2. Calculated energies of the transitions in the absorption spectra of the [VO(O2)(Hheida)]− anion dissolved in water and corresponding oscillator strengths. № 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Energy, cm-1 22589 34380 37781 38609 38588 43344 43763 46700 44609 46662 46485 46362 48266 49981

Oscillator strength 0.019 0.020 0.027 0.011 0.013 0.018 0.011 0.012 0.008 0.009 0.013 0.012 0.019 0.011

range 200-600 nm the calculated spectrum consists of three bands that approximately extend in the regions: (a) 200-231 nm; (b) 259-291 nm; (c) 380-520 nm. The analysis of the results of DFT calculations shows (Table 2) that the highest in energy band is formed by the transitions with the energies 43344 cm-1, 43763 cm-1, 46700 cm-1, 44609 cm-1, 46662 cm-1, 46485 cm-1, 46362 cm-1, 48266 cm-1 and 49981 cm-1. The most intensive transitions occur at 43344 and 48266 cm-1, their oscillator strengths almost double the oscillator strengths of other transitions in the range 200-231 nm. The assignment of the transitions given in Table 1 was performed as following. First, using the ORCA program the partial transitions between the molecular orbitals that form the transition at a definite energy (Table 2) have been analyzed. For instance, from ORCA calculations it follows that the transition at 48266 cm-1 is mainly formed by transitions 87a→110a and 107a→118a, where 87a, 110a, 107a and 118a label the molecular orbitals of the complex under examination. Then with the aid of ORCA program for each molecular orbital participating in the transition we obtained its



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composition. This allowed to understand the origin of the molecular orbital and, namely, to clarify whether this orbital is mainly of the ligand or metal type. In such a way we determined the origin of the partial transitions forming a definite transition listed in Table 2 and answered the question what type of transfers forms this transition. The analysis of the electron transfers between the molecular orbitals contributing to the transitions at 48266 and 43344 cm-1 shows that both transitions can be assigned to ligand–metal electron transfer of the type of O→V. In these transitions the contribution of the electron transfer from the O1 ligand (Fig.1) that belongs to the double bond oxogroup to the vanadium ion is dominating. At the same time the transfers O4→V, O8→V, N→V also noticeably contribute to the calculated band with the maximum at 214 nm. The calculated band with the maximum at 265 nm mainly originates from the transitions with the energies 34380 and 37781 cm-1 and oscillator strengths 0.020 and 0.027, respectively. The analysis of the composition of the molecular orbitals shows that the mentioned transitions mainly refer to the excitation O8→V.

Finally, the broad absorbance band with the maximum at ∼430 nm that solely originates from the

transition at 22589 cm-1 with the oscillator strength 0.019 refers to the electron transfer from the

peroxo group, and, namely, from the O3 ligand to the central vanadium ion. This confirms the identity of a vanadium peroxo group in the examined complex. Finally, it should be underlined that the overlap of the bands corresponding to the close in energy individual transitions with oscillator strengths of the order of 0.01-0.02 (see Table 2) gives the observed spectra with the pronounced ligand-metal charge transfer band and the peroxo band much smaller in intensity.

4.2. INFRARED SPECTRA The observed IR spectrum of the K[VO(O2)Hheida] complex in the range of 400-3600 cm-1 obtained by the ATR method is shown in Fig.S5 of the section “Supporting Information”. Conventionally this spectrum can be subdivided into 7 groups of relatively intensive peaks that fall into the ranges 400-650 cm-1, 700-1170 cm-1, 1200-1475 cm-1, 1480-1725 cm-1, 1795-2300 cm-1, 2400-3150 cm-1 and 3160-3600 cm-1, respectively. Since the main vibrations of water fall into the range 3000-4000 cm-1 the presence of water in the K[VO(O2)Hheida]⋅(H2O) sample is testified by Fig.1. For the K[VO(O2)(Hheida)] complex dissolved in water the calculated IR spectrum in the range 400-1100 cm-1 is illustrated in Fig.3. The spectrum was obtained with the aid of ORCA package 17 (Keyword NumGrad used for calculation of the numerical frequencies). The frequencies belonging to the vibrations of the organic groups in the [VO(O2)(Hheida)]− complex and those belonging to water are not shown. In the range of 420-599 cm-1 the magnitudes of the overwhelming majority of calculated frequencies are in satisfactory agreement with the experimental ones. The


Page 13 of 21

difference between 6WX# and 6YZ[Y is of the order of several wavenumbers, the intensities of the

869

1044

1001 924

900

b)

971

883

1056

1068

1013

819

742 750

1000

1096

599

450

413

428

958

905

586

512 467

437 442 404

420

642

548 565

922

991

818

712

545 566

516

438

1022

806

720 740

613 628 599

446 463

424

825

582

525

484

555

950

1066

967 980

703

observed and calculated frequencies are also of the same order of magnitude.

Absorbance / a.u.

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


a)

400

500

600

700

800

Wavenumbers, cm

900

1000

1100

-1

Fig.3.Observed IR spectra of K[VO(O2)(Hheida)]·H2O (a) and calculated spectra of the [VO(O2)(Hheida)]-anion (b) dissolved in water. The largest discrepancy of 14 cm-1 in this frequency range is obtained between the highest in energy mode observed at 642 cm-1 and the calculated value of 628 cm-1. The computed frequencies with the energies 740cm-1, 818 cm-1, 869 cm-1, 900 cm-1 and 924 cm-1 also fit quite nice the observed ones at 742, 819, 883, 905 and 922 cm-1. As to the group of frequencies calculated in the range of 950-1066 cm-1 it also resembles the observed ones in the interval 958-1068 cm-1. Thereby, besides



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the obtained satisfactory agreement between the positions and the intensities of the calculated and experimental frequencies the performed modeling of the IR spectra shows a correct grouping of the frequencies that resembles the experimental one. Thus, the calculated spectrum for the [VO(O2)(heida)]− anion reproduces the main features of the experimental one testifying that in K[VO(O2)(Hheida)] the effect of potassium on the main vibrations involving the vanadium ion and its nearest surrounding is not so strong. Concerning the observed and calculated vibrations one more remark should be made. Both the experimental and calculated spectra contain a greater number of vibrations than it is shown in Figs 3a and 3b. However, the intensities of several vibrations are much weaker in comparison with those illustrated in the figures and, therefore, these vibrations do not manifest themselves evidently in the spectra. Meanwhile, their contribution to the apparent absorption spectra can be appreciable. Therefore, the calculated apparent absorption spectrum accounts for all calculated vibrations. The second important question to be discussed is the influence of water on the vibrational spectra of the K[VO(O2)(Hheida)]·H2O compound and the [VO(O2)(Hheida)]− anion dissolved in water. As it was already mentioned the measurements of the IR spectra by the ATR method have been performed on a K[VO(O2)(Hheida)] sample containing one water molecule per unit cell that gives an appreciable peak observed in Fig.S5. From this it follows that the influence of water on the IR spectrum of this compound cannot be excluded. From the other side, the DFT calculations of the vibrational frequencies for the [VO(O2)(Hheida)]− anion dissolved in water were performed in the conductor-like screening model (COSMO)

30

which approximates the solvent by a dielectric

continuum surrounding the solute molecules outside of the molecular cavity. This allows to suppose that the water continuum surrounding the [VO(O2)(Hheida)]- anion as well as 8 water molecules explicitly included in the model cannot affect appreciably the frequencies of the vibrations involving the vanadium ion and its nearest surrounding. Moreover, the damping effect of water on the force constants determining the frequencies is comparatively small. Taking into account these considerations one can justify the performed comparison of the frequencies calculated for the [VO(O2)(Hheida)]- anion dissolved in water with those measured for the K[VO(O2)(Hheida)]·H2O compound. To understand how during a definite vibration the bonds between the atoms move and the configuration of the molecule changes for the [VO(O2)(heida)]− anion dissolved in water the visualization of the calculated vibrations in the energy range of 400-1100 cm-1 has been performed with the aid of the program Chemcraft. 31 Further on we only describe the motion of atoms in those vibrations in which the central vanadium atom and its nearest surrounding are involved. The motion of atoms in these vibrations is schematically shown in Fig.4. In the vibrations at 484 and 516 cm-1 the distance between the V and N atoms mainly changes in time (Fig.4a). The vibrations at 545, ACS Paragon Plus Environment

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555, 599 and 613 cm-1 (Figs. 4b, 4c) are characterized by the motion of the peroxo group as a whole, in these vibrations the length of the bond O2-O3 remains constant, while the bonds V-O2 and V-O3 change in time. However, in the vibrations with the energies 545 and 555 cm-1 the oxygens O2 and O3 move in opposite directions (Fig. 4b), while in the modes 599 and O1

N O4

V

V

V

O6 O3

O3

O2 H

O3 O2

O2

O8

a)

b)

c)

N O4

V

V

O4

V

O4 O6

d)

O6

e)

f)

O1 O1 N N

V

O3

V

O4

V

O3

O2

O2 O8

H O8 H

g)

h)

i)

Fig.4. Schematic illustration of the vibrations of the [VO(O2)(heida)]- anion dissolved in water corresponding to following calculated frequencies: (a) 484 cm-1, 516 cm-1; (b) 545 cm-1, 555 cm-1; c ) 599 cm-1, 613 cm-1; d) 582 cm-1; e) 900 cm-1; f) 924 cm-1; g) 950 cm-1; h) 980 cm-1, 991 cm-1; i) 1001 cm-1, 1022 cm-1, 1044 cm-1



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613 cm-1 (Fig.4c) the displacements of these oxygens are parallel. The vibration at 582 cm-1 (Fig. 4d) is mainly characterized by the decrease and increase of the magnitude of the angle ∠O4-V-N.

The change in the distances V-O4 and V-O6 is characteristic for the vibrations at 900 and 924 cm-1 (Figs 4e, 4f), respectively. The more complicated is the vibration with energy 950 cm-1 (Fig.4g). It is characterized by the change in the bonds V=O1 and O2-O3. Moreover, the increase in the distance V=O1 is accompanied by the reduction of the bond length O2-O3 in the peroxo group and vice versa. The vibrations with frequencies equal to 980 and 991 cm-1 (Fig. 4h) include the

modulations of the distances V=O1, O2-O3, V-N as well as of the angle ∠V-O8-H. The main

feature of the higher in energy vibrations at 1001, 1022 and 1044 cm-1 (Fig. 4i) is the modulation of

the angle ∠V-O8-H. The vibrations at 1022 and 1044 cm-1 also lead to the change in time of the angles ∠O4-V-N and ∠O8-V-N, respectively.

CONCLUDING REMARKS In the present paper a DFT study of the apparent absorption and IR spectra of the [VO(O)Hheida]− complex dissolved in water is reported. At the beginning the optimization of the complex structure, calculation of the vibrational frequencies as well as of the transition energies and absorption profiles have been performed completely with the aid of the ORCA program. 16,17 These calculations apparently showed that the values of the inhomogeneous line width θ p (Eq. (1)) higher than 1000 cm-1 do not change the shape of the band. Secondly, the absorption spectrum of the [VO(O2)Hheida]− complex was calculated in the approximation of “strong heat release” with the aid of the formulae (4)-(8) obtained in

37

(see also Refs. [33-35]). In this approach the parameters

0 ΩG #$ , J0# and "#$ determining the absorption band were taken from DFT calculations as well. At

the same time the shape of each individual %→' transition was taken as Gaussian one as it follows from [37]. It was obtained that the curves calculated in two different ways coincide. This means that in fact for widths θ p >1000 cm-1 within the ORCA package the approximation of “strong heat release” works. However, both approaches give a blue shifted position of the peroxo band. To achieve a better agreement between the calculated curve and the experimental data the constants

Δ*# =

LM$NOP N$QM#NOPN#QR ℏTP

corresponding to the vibrational frequencies 582 and 591 cm-1 and

representing in fact the difference of the coordinates of the minima of the adiabatic potential sheets in the ground % and excited ' states should be insignificantly increased. After adjusting of the mentioned Δ*#

values the difference between the observed and calculated curves improves

evidently. Since the ORCA package allows to take into account the solvent effects only on the first two stages of the optical band calculations by using the option “Cosmo” to overcome this deficiency in our model several water molecules have been placed in the vicinity of the V atom and ACS Paragon Plus Environment

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its nearest surrounding. The accepted model also describes quite well the positions, intensities and the grouping of frequencies in the NIR spectra.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Optimized structures and apparent absorption spectra of the [VO(O2)(Hheida)]− complex dissolved in water calculated with inclusion of a different number of additional water molecules in the model, infrared spectra of the K[VO(O2)(Hheida)] compound in the range 400 -3700 nm obtained by ATR method.

ACKNOWLEDGEMENT SK and OR are grateful to the Max-Planck Gesellschaft ( Germany) and Supreme Council for Science and Technological Development of the Republic of Moldova (project 15.817.02.06F) for financial support.

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Leblanc, C.; Vilter, H.; Fournier, J. B.; Delage, L.; Potin, P.; Rebuffet, E.; Michel, G.; Solari, P. L.; Feiters, M. C.; Czjzek, M. Vanadium Haloperoxidases: From the Discovery 30 Years ago to X-ray Crystallographic and V K-edge Absorption Spectroscopic Studies. Coord. Chem. Rev. 2015, 301, 134-146.

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Chrappova, J.; Schwendt, P.; Sivak, M.; Repisky, M.; Malkin,V. G.; Marek, J. Dinuclear Fluoro-Peroxovanadium(V) Complexes With Symmetric and Asymmetric Peroxo Bridges: Syntheses, Structures and DFT Studies. Dalton Trans. 2009, 465-473.

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(10) Coggins, M. K.; Martin-Diaconescu, V.; DeBeer, S.; Kovacs, J. A. Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes. J. Am. Chem. Soc. 2013, 135, 42604272. (11) Sun, L.L.; Hermann, K.E.; Noack, J.; Timpe, O.; Teschner, D.; Hävecker, M.; Trunschke, A.; Schlögl, R. DFT Studies and Experiments on Biocatalytic Centers: Structure, Vibrations, and Core Excitations of the K[VO(O2)Hheida] Complex. J. Phys. Chem. C 2014, 118, 2461124622. (12) Maganos, D.; Trunschke, A.; Schlögl, R.; Neese, F. A Unified View on Heterogeneous and Homogeneous Catalysts Through a Combination of Spectroscopy and Quantum Chemistry. Faraday Discuss. 2016. 188, 181-197. (13) Jacob, A. S.; Lakshman, S.V. Electronic Spectra of VO2+ in Some Tutton Salts. J. Mat. Let.,

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1963, 80, 553. (33) Perlin, Yu.E.; Tsukerblat, B.S. Optical Bands and Polarization Dichroism of Jahn-Teller Centers, in: The dynamical Jahn-Teller effect in localized systems. vol. 7; Yu.E. Perlin and M. Wagner, Eds., Elsevier, Amsterdam, 1984. (34) Perlin, Yu. E.; Tsukerblat, B.S. Effects of Electron-Vibrational Interactionin in the Optical Spectra of Impurity Ions; Ştiinţa, Kishinev, 1974. (35) Petrenko, T.; Krylova, O.; Neese, F.; Sokolowski, M. Optical Absorption and Emission Properties of Rubrene: Insight From a Combined Experimental and Theoretical Study. New J. Phys, 2009, 11, 015001 (23 pages). (36) Runge, E.; Gross, E. K. U. Density-Functional Theory for Time-Dependent Systems. Phys. Rev. Lett. 1984, 52, 997-1000. (37) Krivoglaz, M. A.; Pekar, S. I. Shape of Impurity Absorption and Luminescence Spectra in Dielectrics. Proceedings of the Institute of Physics of AN USSR, 1953, 4, 37-70.


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TOC Graphic 1.0

Absorption, a.u.

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0.8 0.6 0.4 0.2

calculated

0.0 200

observed

300

400

λ, nm

500

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700


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