Hydrogen bonding and solvent effects on the lowest 1(n, π*) excitations of triazines in water

Hydrogen Bonding and Solvent Effects on the Lowest1(n, �*) Excitations of Triazines in Water

JUN ZENG,1,2 DAIQIAN XIE3

1Department of Biochemistry, La Trobc University, Bundoora, Vic 3086, Australia2Cytopia Pty Ltd, 5th Floor, the Baker Heart Research Institute, Commercial Road,

Melbourne, Vic 3004, Australia3Institute of Theoretical and Computational Chemistry, Department of Chemistry,

Nanjing University, Nanjing, 21003, People’s Republic of China

Received 26 September 2003; Accepted 26 December 2003

Abstract: Our method for estimating solvent effects on electronic spectra in media with strong solute–solventinteractions is applied here to calculate the absorption and fluorescence solvatochromatic shifts of dilute triazines inwater. First, the ab initio CASSCF method is used to estimate the gas-phase electronic excitation properties and statecharge distributions; second, Monte Carlo simulations are performed to elucidate liquid structures around the ground andexcited state solute; finally, the solvent shift is evaluated based on the gas-phase charge distributions and the explicitsolvent structures. For the dilute triazine solutions, simulations predict one linear (different) hydrogen bond attached toeach nitrogen atom. Upon the first 1(n, �*) electronic excitation one hydrogen bond is completely broken. For theabsorption and fluorescence spectra, our calculations demonstrated that the specific solvent–solute interaction, in anyelectronic state, plays a critical role in the determination of solvent shifts.

© 2004 Wiley Periodicals, Inc. J Comput Chem 25: 813–822, 2004

Key words: solvent effect; triazines; electronic spectroscopy; computer simulations

Introduction

Electronic absorption and fluorescence spectroscopy has beenwidely used to study the electronic properties of materials in thecondensed phase. In a polar media, sizable electrostatic solvent–solute interactions can induce a large solvatochromic shift on anelectronic absorption or fluorescence band center. Such specificinteractions play a critical role in many electronic transfer pro-cesses in solution.1 Previously, we have developed a method forstudying specific solvation effects of solvents on chromosphoressuch as azines)2–8 and inorganic complexes.9–12 Briefly, themethod consists of two steps: first, to use conventional simulationtechniques such as Monte Carlo or molecular dynamics to generatesufficient equilibrated configurations for the system, and second,select 1000 of these configurations for the solvent shift determi-nation. In the second step, we choose to ignore the dispersioneffects and charge transfer to solvent because they are usually ofquite small magnitude (the order of 300 cm�1) in most of thecharge transfer systems. Polarizable molecular charge distributionsof the solvent and the solute are used in both initial and finalelectronic states, and the solvent shift is calculated as the differ-ence of the electrostatic solute–solvent interaction upon an elec-tronic excitation.5

In principle, one can use any viable scheme for the first step togenerate the liquid structure around the solute. We have adopted theKollman’s scheme13,14 for the generation of intermolecular pair po-tentials and used these in a Monte Carlo simulation of solutions.Although this method is intended for calculations of interaction po-tentials between molecules in their ground electronic states only, wehave found that it can be applied to describe the interactions betweenmolecules in different electronic states.4,6–8 The potential containstwo parts: atom-type–based interatomic Lennard–Jones interactions,and molecule-specific electrostatic interactions. In the standard form,the electrostatic component of the potential is parameterized in termsof ab initio self-consistent field (SCF) atomic point charges obtainedby fitting the ab initio molecular electrostatic potential (ESP). Inprevious studies on azine solutions, we have developed and applied amodified form in which atomic point charges and dipoles are used, aswell as some nonatomic point charges and dipoles.5 The advantage of

Correspondence to: J. Zeng; e-mail: [email protected]

Contract/grant sponsor: National Science Foundation of China; Contract/grant number: 30370337 (to D. X.)

Contract/grant sponsor: Teaching and Research Award Program forOutstanding Young Teachers in Higher Education Institutions of MOE,China.

© 2004 Wiley Periodicals, Inc.

the revised scheme is that it provides a significant improved descrip-tion of the ab initio potential, which is most noticeable around thehydrogen bond-forming regions near azine nitrogen atoms.5–7 Unfor-tunately, the Lennard–Jones parameters used in Kollman’s schemeare not optimized for this type of charge distribution, and as a result,intermolecular potentials obtained in this fashion always overestimatethe hydrogen bond strength.5–7 Although the original scheme hasprovided the most realistic solvent structures in many studies, in somespecific cases where two or more nitrogen atoms are adjunct (e.g.,pyridazine and 1,2,3-triazine), the revised scheme provides moreappropriate hydrogen bonding structure of solvent to the nitrogenatoms.7,8 Therefore, we apply the revised scheme for description ofelectrostatics of triazines in this study.

We attempt to interpret the observed solvent shifts of the lowest1(n, �*) excitation of triazines in dilute aqueous solution. Previously,we have studied solvent shifts of 1(n, �*) absorption and fluorescencespectra of dilute pyridine and diazines in water.2–8 This work thus

completes a series of studies on the interpretation of azine absorptionand fluorescence shifts.

Triazines vary significantly in stabilities, symmetries, and aroma-ticity gradation.15,16 There are three different triazine isomers (i.e.,1,2,3-triazine, 1,2,4-triazine, and 1,3,5-triazine) with different sym-metries (Fig. 1). Because of many near lying excited states, hydrogenbonding to the excited-state triazines is of particular interest, makingthem typical examples for elucidation of physical and structuralproperties of hydrogen bonds of solvent around the ground andexcited-state solute.

For each triazine isomer, the first three 1(n, �*) excitationsinclude 1A2(a1 3 a2), 1A2 (b2 3 b1) and 1B1(a1 3 b1) in1,2,3-triazine,17 1A� (a�3 a�), 1A�(a�3 a�) and 1A� (a�3 a�) in1,2,4-triazine,18 and 1a�2(e�3 e�), 1A�1(e�3 e�), and 1E�(e�3 e�)in 1,3,5-triazine.19 Several high-quality ab initio calculations [i.e.,CASSCF, CASPT2, and Multiple Reference Configuration Interac-tion (MRCI)] have been performed to assign the electronic excitationto observed absorption spectrum, with certain variance.17–20 Here, wechose the first 1B1(a13 b1), 1A�(a�3 a�) and 1A�2(e�3 e�) as thelowest 1(n, �*) excitations for the 1,2,3-, 1,2,4-, and 1,3,5-triazine,because only these electronic excitations have large oscillationstrength and can be observed in the condensed phase.

Computational Methods

Liquid Simulations

Convergence-accelerated3,21 rigid-molecule constant number,temperature, and pressure (NPT-ensemble)22 Monte Carlo simu-

Table 1. Cartesian Coordinates in Å, in Terms of the Molecular Normal, Short, and Long Axis for the N, S,L Atoms of Triazines as Well as for an Additional Point Charges X That Are Used in Individual Potentials.

1,2,3-Triazinea 1,2,4-Triazineb 1,3,5-Triazinec

Atomd N S L N S L N S LN1 0.0 �1.1570 0.7356 0.0 �1.1841 0.6935 0.0 �1.1974 �0.6913N2 0.0 0.0 1.3826 0.0 0.0 1.3433 0.0 0.0 1.3826N3 0.0 1.1570 0.7356 0.0 1.2274 �0.7392 0.0 1.1974 �0.6913C4 0.0 �1.1710 �0.6113 0.0 �1.1535 �0.6518 0.0 �1.1157 0.6441C5 0.0 0.0 �1.3514 0.0 0.0490 �1.3750 0.0 0.0 �1.2883C6 0.0 1.1710 �0.6113 0.0 1.1271 0.6063 0.0 1.1157 0.6441H7 0.0 �2.1640 �1.0491 0.0 �2.1198 �1.1437 0.0 �2.0545 1.1861H8 0.0 0.0 �2.4364 0.0 0.0626 �2.4600 0.0 0.0 �2.3723H9 0.0 2.1640 �1.0491 0.0 2.0514 1.1725 0.0 2.0545 1.1861

S0 X10–X11 �0.0309 0.8019 �0.0452 �0.0913 0.4432 1.3158 �0.3988 1.0856 0.8944X12–X13 �0.0309 �0.8019 �0.0452 �0.1581 �1.5089 0.7877 �0.0076 0.9723 �0.3309X14–X15 �0.0227 1.6578 �0.8004 �0.4553 1.7209 0.6069 �0.3988 �1.0856 0.8944X16–X17 �0.0227 �1.6578 �0.8004 �0.2732 �0.9761 0.1326 �0.0076 �0.9723 �0.3309

S1 X10–X11 �0.0239 1.6005 �0.6811 �0.1188 1.4008 0.4959 �0.1206 1.1967 1.3466X12–X13 �0.0239 �1.6005 �0.6811 �0.6130 �0.7807 1.3069 �0.0706 1.1470 0.2165X14–X15 �0.0504 0.6954 0.9303 �0.4215 �1.4105 0.8609 �0.1205 �1.1967 1.3466X16–X17 �0.0504 �0.6954 0.9303 �0.5673 1.2729 �1.2029 �0.0706 �1.1470 0.2165

aX-ray structure.34

bGeometry was optimized from MP2 ab initio calculations using DZP basis set.18

cNMR structure.35,36

dAtom numbers are labeled according to Figure 1.

Figure 1. Geometry of three different forms of triazines in terms ofthe molecular in-plane long (L) and short (S) axis.

814 Zeng and Xie • Vol. 25, No. 6 • Journal of Computational Chemistry

lations23 are performed at a temperature of 298 K and p � 1 atomfor a sample containing one triazine and 102 water molecules. Thenumber of solvent molecules was found to be sufficient to describethermodynamic and spectroscopic properties of dilute azines inwater.3–6,8 Periodic truncated octahedral boundary conditions24–26

are used. For each triazine, equilibration is performed for at least107 moves, followed by 20 � 107 moves of sample configurations.Every 200th configuration is analyzed to determine the radialdistribution functions (rdfs) and every 2000th configuration issubsequently analyzed to determine solvent shifts. Pairwise addi-tive intermolecular potentials are constructed using the modifiedscheme of Kollman’s function form,13,14 which also specifies theTIP3P water potential.27 For the Lennard–Jones interactions, weuse the same parameter set for both the ground and excited states,as described previously.3 Although these Lennard–Jones parame-

ters are derived for the ground state, we have shown that it is alsoapplicable to the 1(n, �*) excited states of azines.4,6–8

The columbic interaction of Kollman’s intermolecular potentialis comprised of the interaction between the TIP3P water chargesand atomic charges of solute, which are determined by fitting3 theab initio electrostatic potential (ESP). We generate the atomic/point charges and dipoles in this fashion for the ground and thelowest 1(n, �*) excited states of three triazines, and will bediscussed later.

Solvent Shift Calculations

The method used to evaluate the solvent shift at the center of theabsorption and fluorescence bands has been described in detailelsewhere.5 It requires as data sample liquid configurations as well

Table 2. Atomic Charges, in e, Used in Each Potential of the Ground (S0) and the Lowest (n,�*) Excited State (S1) of 1,2,3-, 1,2,4-, and 1,3,5-Triazines.

Atom

1,2,3-Triazine 1,2,4-Triazine 1,3,5-Triazine

S0 S1 S0 S1 S0 S1

N1 �0.6100 �0.6682 0.0758 �0.0474 �0.2483 0.2272N2 0.1644 0.6870 �0.1689 �0.1775 �0.2033 0.0071N3 �0.6100 �0.6682 �0.1034 �0.0374 �0.2483 0.2272C4 0.9304 0.5274 0.0098 0.2789 0.0465 �0.2128C5 �1.0076 �0.7729 �0.0908 �0.4336 0.0949 �0.6877C6 0.9304 0.5274 �0.3535 �0.2347 0.0465 �0.2128H7 �0.0445 0.0379 0.2049 0.1869 0.1744 0.2095H8 0.2911 0.2913 0.1942 0.2331 0.1625 0.2321H9 �0.0445 0.0379 0.2314 0.2311 0.1744 0.2095X10–X11 0.4708 �0.0897 0.2313 0.2090 �0.3934 �0.6767X12–X13 0.4708 �0.0897 1.0989 0.5330 0.3935 0.6769X14–X15 �0.4707 0.0898 �0.1870 �0.5895 �0.3934 �0.6767X16–X17 �0.4707 0.0898 �1.1429 �0.1522 0.3935 0.6769

Table 3. Atomic Point Dipoles (in Debyes) in Each Potential.

Atom

1,2,3-Triazine 1,2,4-Triazine 1,3,5-Triazine

N S L N S L N S L

S0 N1 0.0 1.3733 �0.1620 0.0 1.2069 �0.9254 0.0 2.6255 �0.7996N2 0.0 0.0 �2.1485 0.0 �0.5094 �1.0894 0.0 0.0 �1.7400N3 0.0 �1.3733 �0.1620 0.0 �1.3959 0.4600 0.0 �2.6255 �0.7996

X10–X11 �0.3092 �0.2094 0.8418 �1.1011 0.6827 �0.7398 �0.1356 �0.0700 0.9460X12–X13 �0.2374 �0.2496 0.5383 �1.2122 1.5911 �1.3522 �9.1580 1.1086 2.1695X14–X15 �0.3092 0.2094 0.8418 �0.1604 0.4572 �0.2257 �0.1356 0.0700 0.9460X16–X17 �0.2374 0.2496 0.5383 �1.3753 0.9350 �2.0287 �9.1580 �1.1086 2.1695

S1 N1 0.0 1.6606 �5.4453 0.0 1.2803 �1.2730 0.0 1.8856 0.1126N2 0.0 0.0 0.9009 0.0 �1.0345 1.1502 0.0 0.0 1.0844N3 0.0 �1.6606 �5.4453 0.0 �2.2286 �0.0300 0.0 �1.8856 0.1126

X10–X11 �0.7638 0.6362 �0.4130 �2.5383 �0.6204 �0.6441 �1.3217 0.3863 1.3853X12–X13 �0.8011 �0.0314 �0.2648 �0.4345 �1.0350 �0.700 �4.6161 �0.4771 2.2698X14–X15 �0.7638 �0.6362 �0.4130 �0.1987 �0.8664 �0.5140 �1.3217 �0.3863 1.3853X16–X17 �0.8011 0.0314 �0.2648 �0.0592 0.0566 �0.4269 �4.6161 0.4771 2.2698

Hydrogen Bonding Effects of Excitations of Triazines 815

as a representation of the solute’s initial and final-state electrostaticpotential and polarizabilities. The polarizabilities of triazine arecalculated using an SCF level ab initio calculation via the GAUSS-IAN package.28 The resulted ground-state polarizabilities are(�NN, �SS, �LL) � (22, 54, 52) a.u., (20, 54, 53) a.u., and (20, 51,51) a.u. for 1,2,3-triazine, 1,2,4-triazine, and 1,3,5-triazine, respec-tively, and the polarizabilities for the excited state are (19, 48, 51)a.u., (19, 53, 62) a.u., and (20, 13, 63) a.u., respectively.

During the solvent-shift evaluations, the molecular parametersfor water molecules are defined as: gas-phase atomic charges qH �0.33e, qO � �0.66e,5 dielectric constant � � 78.5,29 refractiveindex n � 1.333,29 and isotropic polarizability � � 9.6164 a.u.30

For calculating the solvent shift of a vertical excitation, we usespherical boundary conditions and Friedman image charge31 tech-nology to model the very long-range electrostatic interactions. Weignore the hyperpolarizability and molecular charge-transfer pro-cesses, as they are believed to be unimportant for azines in water,and the dispersion interactions, as they are very expensive toevaluate and contribute only a magnitude of a few hundred wavenumbers to the solvent shift.5

Gas Phase Electronic Structure Calculation

Previously, CASSCF calculations have been shown to be capableof reproducing the relative energies of the (n, �*) excitations oftriazines.17–20 Here, we perform the CASSCF ab initio electronicstructure calculations to determine the properties of the ground(S0) and the first 1(n, �*) excited states (S1) of three triazines. Adouble-zeta plus polarization (DZP) basis sets32 containing 105basis functions is used for 1,2,3-, 1,2,4-, and 1,3,5-triazine via theHONDO33 program at their observed ground-state equilibriumgeometries (Table 1 and Fig. 1; refs. 34–36). However, the exper-imental geometry of 1,2,4-triazine is not available; thus, its geom-etry is optimized at the DZP-MP2 level. The results are in goodagreement to the previous ab initio calculations using differentbasis sets.18

The active spaces are chosen differently for CASSCF calcula-tions for different triazines. For the (45a1, 11a2, 16b1, 33b2)orbitals in the C2v symmetry of 1,2,3-triazine, (3, 2, 3, 2) specifiesthe active space into which 10 electrons are distributed. The activespace was optimized for (n, �*) states and consists of five

nonbonded orbitals (a1, b2, a1, b2, a1) and five lowest antibonding�* orbitals (b1, a2, b2, b1, b1). For the (78a�, 27a�) orbitals in theCs symmetry of 1,2,4-triazine, the active space consists of fivenonbonded orbitals 5a� and five lowest antibonding �* orbital 5a�for 10 electrons. For the (19a�1, 2a�1, 7a�2, 26e�, 7e�) orbitals of theD3h symmetry of 1,3,5-triazine, (1, 1, 1, 2, 5) is defined as theactive space to accommodate the singly and doubly (SD) exci-tations of 10 active electrons. As a result, the relative energiesfor the lowest (n, �*) excitations of 1,2,3-, 1,2,4-, and 1,3,5-triazine are calculated to be 4.42, 4.29, and 4.57 eV, close toprevious ab initio results (1,2,3-triazine: 4.1017 and 4.56 eV,20;1,2,4-triazine: 3.56 eV,18; 1,3,5-triazine: 4.24eV19). It is notice-able that the comparison of the computational results with theobserved values indicates a significant uncertainty. Althoughthe CASSCF calculations presented here and reported previ-ously overestimate the relative excitation energies by 0.23–0.3020 for 1,2,3-triazine and 0.46 eV for 1,2,4-triazine,18 theprevious CASPT2 calculations20 underestimate the 1,2,3-tri-azine 1B1 excitation energy by almost 1.0 eV. For 1,3,5-tri-azine, the excitation energy from our CASSCF calculation is inagreement with the experimental value of 4.58 eV.19

The CASSCF wave functions are subsequently used to evaluateelectrostatic potentials at test points generated over the region ofthe first few solvation shells. The points are specifically locatedbetween 1.4 and 2.5 times the molecular van der Waals shell: formodeling the specific hydrogen-bonding interactions, we useslightly modified radii of 1.3, 1.7, and 1.2 Å for N, C, and Hatoms.3 The modified scheme of ESP charges is derived fromfitting the electrostatic potentials of triazines, and consists of threecomponents: atomic charges of triazines, atomic point dipole con-tribution on each nitrogen atom that explicitly describes the largecontributions to the molecular dipole moment from local s-phybridization, and two symmetry-related sets of floating pointcharges and dipoles, named X10–X13 and X14–X17, that was used

Table 4. Resulting Root-Mean-Square Error, in kcal/mol/e, of Fitting theAb initio ESP Potential, as Well as the Dipole Moment (in Debyes)Calculated from the ESP Model, and Electronic Wave FunctionCalculations.

Surface ESP error �(ESP) � (CASSCF)� (abinitio)a

1,2,3-Triazine(S0) 0.16 4.98 4.97 5.271,2,3-Triazine(S1) 0.30 2.71 2.691,2,4-Triazine(S0) 0.21 2.70 2.70 2.751,2,4-Triazine(S1) 0.23 0.36 0.371,3,5-Triazine(S0) 0.19 0.0 0.0 0.01,3,5-Triazine(S1) 0.29 2.46 2.48

aObtained from MP2 calculations using TZVP basis set.17

Table 5. Results from Liquid Simulations.a,b

S0 S1

1,2,3-triazine�H �21.67 �16.06�V 62.00 57.64Coord no.c 3 2



a�H � enthalpy of hydration, in kcal/mol, with an uncertainty of �1.5kcal/mol, �V � partial specific volume, in ml/mol, with an uncertainty of�5 ml/mol.bBased on the observed results of diazines38 and sym-triazine,37 the exper-imental value of �H and �V of three triazines are deferred to be around�21.45 kcal/mol and 55.0 ml/mol, respectively.cThe number of NOH hydrogen bonds per triazine.


to model the �-ring charge density. With this scheme, reasonablefits to the ab initio ESP are obtained for all the triazine potentialsurfaces, with the root-mean-square errors of the fits to the groundand excited states being (0.16, 0.30) kcal/mol/e for 1,2,3-triazine,(0.21, 0.23) kcal/mol/e for 1,2,4-triazine, and (0.19, 0.29) kcal/mol/e for 1,3,5-triazine (see Table 4). Note that we use the ground-state geometry to obtain the ESP charges of the excited-statetriazines, because the equilibrium structure of triazine in the 1(n,�*) excited state is similar to its ground-state structure as dem-onstrated from previous ab initio CASSCF calculations.20 ESPatomic charges are shown in Table 2 and the fitted point chargesand dipoles are in Tables 2–3.

Results

Results obtained for the structure, spectroscopy, and thermody-namics of water around three triazines in their ground and excitedstates (S0 and S1) are shown in Table 5, and triazines nitrogens orcenters to water hydrogen radial distribution functions g(r) areshown in Figure 2. All the ground state simulations predict averagethree hydrogen bonds per triazine with distinct hydrogen-bondingpeaks in gNOH(r) around 2.0 Å, together with correspondingpeaks in gNOO(r), gCENOO(r) and gCENOH(r). The hydrogen-bond structures are well defined with one (different) water mole-cule attached to each nitrogen atom, as highlighted in Figure 3

Figure 2. Radial distribution functions g(r) for the triazine nitrogen (N) and center of mass (CEN) to water oxygen (O) and hydrogen (H) atomsobtained from the simulations on the ground (S0) and excited state (S1) solutions: (a) 1,2,3-triazine; (b) 1,2,4-triazine; (c) 1,3,5-triazine.


where probability contours of OON distances between one of thethree closest water molecules in each configuration to the two ofthree nitrogen atoms shown. The probability contours showed awidespread of rN2OO and rN3OO values for each rN1OO valueand vice versa, indicating different water molecule hydrogenbonded to different nitrogen atom.

The enthalpy of hydration (�H) are calculated to be �21.45 �1.5, �19.00 � 1.5, and �18.75 � 1.5 kcal/mol for 1,2,3-, 1,2,4-,and 1,3,5-triazine, respectively, while the corresponding partialmolar volume (�V) are 62.0 � 5, 68.0 � 5, and 62.8 �5 mL/mol,respectively. Based on the experimental values of sym-triazine37

and diazines,38 thermodynamic properties of three triazines areinferred to be around �H � �21.45 kcal/mol and �V � 55.0

ml/mol, consistent to our simulation results. Moreover, all thecalculated �Hs are within the range of statistic error from theMonte Carlo simulations, indicating that the thermodynamic prop-erties of the ground-state triazines are determined by the solute–solvent hydrogen-bonding interactions.38

For the equilibrated excited states, simulations predict no hy-drogen bonding interaction to the N2 atom and one well-definedhydrogen bond peak of gNOH(r) at NOH distance of 1.8 Å to eachof the other two nitrogen atoms. In the cases of diazines solutions,the (n, �*) electronic excitations are delocalized over two nitro-gen atoms, so as that significantly weakened hydrogen bonds arefound in equilibrated excited-state solutions. The (n, �*) excitedstates of triazines are thus considered as electronically localized,

Figure 2. (continued)


with solution situations analogous to the ground state diazines. Forexample, the excited-states of 1,2,3- and 1,3,5-triazine are analo-gous to the ground state of pyrimidine; the calculated �H in theexcited states of triazines are �16.06 and �15.76 kcal/mol, re-spectively, while the �H of pyrimidine is �16.9 � 1.5 and�19.2 � 1.5 kcal/mol, depending on the potential used.3 Excited-state of 1,2,4-triazine is found to be similar to the ground-statepyrazine with �H of �20.45 kcal/mol close to �18.5 � 1.5kcal/mol obtained from the pyrazine simulations.6

Calculated absorption and fluorescence solvent shifts aregiven in Table 6, together with the solvent shifts of the absorp-tion band origin obtained by substituting the enthalpies ofhydration in the ground and excited states.4,43 The resulted band

origin shifts 1960 cm�1 in 1,2,3-triazine, �290 cm�1 in 1,2,4-triazine, and 730 cm�1 in 1,3,5-triazine, respectively. The cal-culated absorption solvent shifts are 3180, 2920, and 3510cm�1 for three triazines. Experimentally, the absorption spec-trum of triazines in aqueous solutions is not available. For1,2,3-triazine, comparison of the lowest 1(n, �*) absorptionspectrum in vacuum17 and in methanol39 indicates that theabsorption solvent shifts in dilute 1,2,3-triazine in water islikely to be between 2800 an 4100 cm�1. However, it should benoted that the significant coupling between the vibronic andhydrogen bond structures of the lowest 1(n, �*) excited statewas predicted in the 1,2,3-triazine solutions,39 so as that theexact value of the solvent shifts of triazines are very difficult to

Figure 2. (continued)


be measured. Generally, shift of ca. 3000 cm�1 is typical forazines, which is consistent to our calculation.

By reducing the value of the solvent cavity radius r from itsmaximum value (the radius of the inscribed sphere for the unit cell

used in the simulations), it is possible to isolate particular contri-butions to the solvent shifts. Results are shown in Figure 4. Forr � 3.5 Å, no water molecules are included, and the curveshows the r�3 dependence typical of a dipole in a dielectric cavity.

Figure 3. Probabilities of finding hydrogen-bonding pattern between three nitrogen atoms of triazines and water molecule in the simulated liquidstructures.

Table 6. Calculated Solvent Shifts of Band Center and Band Originin cm�1.

Solute LiquidElectron

excitation �UCENTERa,b �UOrigin

1,2,3-Triazine(abs.) S0 S0 3 S1 3180 19601,2,3-Triazine(flu.) S1 S1 3 S0 7601,2,4-Triazine(abs.) S0 S0 3 S1 2920 �2901,2,4-Triazine(flu.) S1 S1 3 S0 �20801,3,5-Triazine(abs.) S0 S0 3 S1 3510 7301,3,5-Triazine(flu.) S1 S1 3 S0 �700

aPreviously, we have found this procedure, for reasonably small solventshifts of band centers such as this, is accurate to ca. 700 cm�1.5bBased on previous studies of 1,2,3-triazine in gas phase17 and methanol,20

the experimental value of absorption shifts of 1,2,3-triazine is estimated tobe between 2800 and 4100 cm�1. The electronic spectrum of 1,2,4- and1,3,5-triazine in aqueous solutions has not been detected.

Figure 4. Calculated absorption shifts as function of cavity radiusused: (F) 1,2,3-triazine (■) 1,2,4-triazine; (Œ) 1,3,5-triazine.


The sharp increase near 4 Å is due to the inclusion of threehydrogen-bonded water molecules. Beyond this dielectric solva-tion of the solute dipole moment by the solvent brings a gradualchange in the solvent shift, until r � 6.5 Å, when another sharpincrease occurs due to the inclusion of the second hydration shellwhere water molecules hydrogen bonded to the water moleculesattached to the nitrogens. However, the second sharp increase doesnot exist in the 1,2,4-triazine solution. Due to its low symmetry(Cs), the (n, �*) electronic excitation causes partial charge trans-fers from N2 to N1. The electrostatic interactions between thesecond hydration shell solvent and the two nitrogen atoms thuscompensate each other, resulting in a smooth curve beyond the firsthydration shell. Overall, the specific hydrogen bonding interac-tions contribute ca. 2500–3000 cm�1 to the total absorption sol-vent shifts of triazines.

For the fluorescence, the calculated solvent shifts vary sig-nificantly among three triazines. While 1,2,3- and 1,3,5-triazinehave a similar magnitude of fluorescence shift with oppositesigns, 1,2,4-triazine has its fluorescence band center red shiftedat 2000 cm�1 in water. For the 1,2,3- and 1,3,5-triazine, thechanges of dipole moment are �2.28 D and 2.48 D upon an 1(n,�*) electronic excitation. Hence, dielectric solvation40 – 42 pro-duces an opposite and similar fluorescence shift of ca. 700cm�1. For 1,2,4-triazine, the excited state is analogous to theground state of pyrazine with two hydrogen bonds attached totwo symmetrically opposite nitrogen atoms. The fluorescenceshift of 1,2,4-triazine is thus expected to be reversed to theabsorption shift of localized excitation of pyrazine (1800 cm�1)in which the specific hydrogen bonding determines the solventshifts.6

Conclusions

Following our previous investigation of (n, �*) absorption andfluorescence shifts of dilute diazines in water,2–8 here we haveapplied our method to analyze solvent shifts of the lowest 1(n, �*)excitations of 1,2,3-, 1,2,4-, and 1,3,5-triazines. The liquid simu-lations on the ground state indicate a well-defined (different) linearhydrogen bond attached to each nitrogen atom, consistent with theenthalpy of hydration argument37,38 in which each azine nitrogenforms a hydrogen bond to solvent in aqueous solutions. Usingthese liquid configurations, our solvent shift calculations presentedhere predict 1(n, �*) band centers of three triazines blue shifted3100, 2900, and 3500 cm�1, in which the calculated shift of1,2,3-triazine is consistent to the observed value.

In the lowest 1(n, �*) excited states, our simulations alsoindicate that two hydrogen bonds to two nitrogen atoms (N1 andN3) are well preserved, and the electronic excitation only breaksthe hydrogen bond to nitrogen atom N2 (Fig. 2). The characteris-tics of the hydrogen-bond structure in the excited states are anal-ogous to the ground states of diazines (i.e., 1,2,3-triazine vs.pyrimidine, 1,2,4-triazine vs. pyrazine, and 1,3,5-triazine vs. py-rimidine), and the resulted thermodynamic properties are thussimilar to the values observed in the simulation of aqueous diazinesolutions.3,4,6 However, these excited-state liquid structures pro-duce significant variance in the calculated fluorescence shifts. In

particular, the specific hydrogen bonding interaction to nitrogenatoms N1 and N2 in the excited state of 1,2,4-triazine produce alarge fluorescence shift of ca. �2000 cm�1, in contrast to the smallmagnitude of solvent shifts calculated for 1,2,3- and 1,3,5-triazine,as well as for the diazines.2,5–7

Together with our previous applications of our method onthe azine aqueous solutions,2– 8 this study completes our inves-tigation on the azine solutions. Clearly, the details of thehydrogen bonding to the chromophore in any electronic state,either ground or excited states, play a critical role on thedetermination of the solvent shifts of electronic spectra. For thefluorescence spectra, the specific interactions could cause theband center shift up to few thousand wave numbers. The liquidstructure must be considered at a molecular level when specificsolvent–solute interactions occur.

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Hydrogen bonding and solvent effects on the lowest 1(n, π*) excitations of triazines in water