The relative position of La and Lb ππ* electronic states in purine nucleobases is a much debated topic, since it can strongly affect our understanding of their photoexcited dynamics. To assess this point, we calculated the absorption and magnetic circular dichroism (MCD) spectra of adenine, guanine, and their nucleosides in gas-phase and aqueous solution, exploiting recent developments in MCD computational technology within time-dependent density functional theory. MCD spectroscopy allows us to resolve the intense S0 La transition from the weak S 0Lb transition. The spectra obtained in water solution, by using B3LYP and CAM-B3LYP functionals and describing solvent effect by cluster models and by the polarizable continuum model (PCM), are in very good agreement with the experimental counterparts, thus providing direct and unambiguous evidence that the energy ordering predicted by TD-DFT, La < L b, is the correct one. © 2014 American Chemical Society.

*** IBB - CNR ***

Istituto di Chimica Dei Composti Organometallici (ICCOM-CNR), Area della Ricerca Del CNR, via Moruzzi 1, I-56124 Pisa, Italy

Istituto di Biostrutture e Bioimmagini-CNR, Via Mezzocannone 6, I-80134 Napoli, Italy

Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden

Dipartimento di Scienze Chimiche e Farmaceutiche, Università Degli Studi di Trieste, via L. Giorgieri 1, I-34127 Trieste, Italy

Crespo-Hernández, C.E., Cohen, B., Hare, P.M., Kohler, B., Ultrafast Excited-State Dynamics in Nucleic Acids (2004) Chem. Rev., 104, pp. 1977-202

Middleton, C.T., De La Harpe, K., Su, C., Law, Y.K., Crespo-Hernández, C.E., Kohler, B., DNA Excited-State Dynamics: From Single Bases to the Double Helix (2009) Annu. Rev. Phys. Chem., 60, pp. 217-239

Gustavsson, T., Improta, R., Markovitsi, D., DNA/RNA: Building Blocks of Life under UV Irradiation (2010) J. Phys. Chem. Lett., 1, pp. 2025-2030

Gonzáles, L., Escudero, D., Serrano-Andres, L., Progress and Challenges in the Calculation of Electronic Excited States (2012) ChemPhysChem, 13, pp. 28-51

Serrano-Andrés, L., Merchán, M., Are the Five Natural DNA/RNA Base Monomers a Good Choice from Natural Selection?: A Photochemical Perspective (2009) J. Photochem. Photobiol., C, 10, pp. 21-32

Matsika, S., Krause, P., Nonadiabatic Events and Conical Intersections (2011) Annu. Rev. Phys. Chem., 62, pp. 621-643

Szalay, P., Watson, T., Perera, A., Lotrich, V., Bartlett, R., Benchmark Studies on the Building Blocks of DNA. 1. Superiority of Coupled Cluster Methods in Describing the Excited States of Nucleobases in the Franck-Condon Region (2012) J. Phys. Chem. A, 116, pp. 6702-6710

Fleig, T., Knecht, S., Hättig, C., Quantum-Chemical Investigation of the Structures and Electronic Spectra of the Nucleic Acid Bases at the Coupled Cluster CC2 Level (2007) J. Phys. Chem. A, 111, pp. 5482-5491

Silva-Junior, M.R., Schreiber, M., Sauer, S.P.A., Thiel, W., Benchmarks for Electronically Excited States: Time-Dependent Density Functional Theory and Density Functional Theory Based Multireference Configuration Interaction (2008) J. Chem. Phys., 129, pp. 104103/1-104103/14

Conti, I., Altoè, P., Stenta, M., Garavelli, M., Orlandi, G., Adenine Deactivation in DNA Resolved at the CASPT2//CASSCF/AMBER Level (2010) Phys. Chem. Chem. Phys., 12, pp. 5016-5023

Picconi, D., Avila Ferrer, F.J., Improta, R., Lami, A., Santoro, F., Quantum-Classical Effective-Modes Dynamics of the ππ* nπ* Decay in 9H-Adenine. A Quadratic Vibronic Coupling Model (2013) Faraday Discuss., 163, p. 223

Gustavsson, T., Sarkar, N., Vaya, I., Jimenez, M., Markovitsi, D., Improta, R., A Joint Experimental/Theoretical Study of the Ultrafast Excited State Deactivation of Deoxyadenosine and 9-Methyladenine in Water and Acetonitrile (2013) Photochem. Photobiol. Sci., 12, pp. 1375-1386

Bisgaard, C., Satzger, H., Ullrich, S., Stolow, A., Excited-State Dynamics of Isolated DNA Bases: A Case Study of Adenine (2009) ChemPhysChem, 10, pp. 101-110

Barbatti, M., Lan, Z., Crespo-Otero, R., Szymczak, J., Lischka, H., Thiel, W., Critical Appraisal of Excited State Nonadiabatic Dynamics Simulations of 9H-Adenine (2012) J. Chem. Phys., 137, pp. 22A503

Kwok, W.-M., Ma, C., Phillips, D.L., Femtosecond Time- and Wavelength-Resolved Fluorescence and Absorption Spectroscopic Study of the Excited States of Adenosine and an Adenine Oligomer (2006) J. Am. Chem. Soc., 62, pp. 11894-11905

Santoro, F., Barone, V., Improta, R., Excited States Decay of the A-T DNA: A PCM/TD-DFT Study in Aqueous Solution of the (9-Methyl-adenine)2·(1-Methyl-thymine) 2 Stacked Tetramer (2009) J. Am. Chem. Soc., 131, pp. 15232-15245

Improta, R., Barone, V., Interplay between "neutral" and "charge-Transfer" Excimers Rules the Excited State Decay in Adenine-Rich Polynucleotides (2011) Angew. Chem., Int. Ed., 50, pp. 12016-12019

Kohler, B., Nonradiative Decay Mechanisms in DNA Model Systems (2010) J. Phys. Chem. Lett., 1, pp. 2047-2053

Lu, Y., Lan, Z., Thiel, W., Hydrogen Bonding Regulates the Monomeric Nonradiative Decay of Adenine in DNA Strands (2011) Angew. Chem., Int. Ed., 50, pp. 6864-6867

Mason, W.R., (2007) A Practical Guide to Magnetic Circular Dichroism Spectroscopy, , Wiley: New York

Solheim, H., Ruud, K., Coriani, S., Norman, P., Complex Polarization Propagator Calculations of Magnetic Circular Dichroism Spectra (2008) J. Chem. Phys., 128, p. 094103

Kjærgaard, T., Jørgensen, P., Thorvaldsen, A., Salek, P., Coriani, S., Gauge-Origin Independent Formulation and Implementation of Magneto-Optical Activity within Atomic-Orbital-Density Based Hartree-Fock and Kohn-Sham Response Theories (2009) J. Chem. Theory Comput., 5, pp. 1997-2020

Kjærgaard, T., Kristensen, K., Kauczor, J., Jørgensen, P., Coriani, S., Thorvaldsen, A., Comparison of Standard and Damped Response Formulations of Magnetic Circular Dichroism (2011) J. Chem. Phys., 135, p. 024112

Kjærgaard, T., Coriani, S., Ruud, K., Ab Initio Calculation of Magnetic Circular Dichroism (2012) WIREs Comput. Mol. Sci., 2, pp. 443-455

Fahleson, T., Kauczor, J., Norman, P., Coriani, S., The Magnetic Circular Dichroism Spectrum of the C60 Fullerene (2013) Mol. Phys., pp. 1401-1404

Djerassi, C., Bunnenberg, E., Elder, D.L., Organic Chemical Applications of Magnetic Circular Dichroism (1971) Pure Appl. Chem., 25, pp. 57-90

Sutherland, J.C., Holmquist, B., Magnetic Circular Dichroism of Biological Molecules (1980) Annu. Rev. Biophys. Bioeng., 9, pp. 293-326

Tomasi, J., Mennucci, B., Cammi, R., Quantum Mechanical Continuum Solvation Models (2005) Chem. Rev., 105, pp. 2999-3093

Becke, A.D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior (1988) Phys. Rev. A, 38, pp. 3098-3100

Lee, C., Yang, W., Parr, R., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density (1988) Phys. Rev. B, 37, pp. 785-789

Dunning, T.H., Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen (1989) J. Chem. Phys., 90, pp. 1007-1023

Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Scalmani, G., Petersson, G.A., (2009) Gaussian 09, , revision C.3

Gaussian Inc. Wallingford, CT

Yanai, T., Tew, D.P., Handy, N.C., A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP) (2004) Chem. Phys. Lett., 393, pp. 51-57

Peach, M.J.G., Helgaker, T., Sałek, P., Keal, T.W., Lutnaes, O.B., Tozer, D.J., Handy, N.C., Assessment of a Coulomb-Attenuated Exchange-Correlation Energy Functional (2006) Phys. Chem. Chem. Phys., 8, pp. 558-562

Woon, D.E., Dunning, T.H., Gaussian Basis Sets for Use in Correlated Molecular Calculations. IV. Calculation of Static Electrical Response Properties (1994) J. Chem. Phys., 100, pp. 2975-2988

(2013) DALTON, , http://daltonprogram.org/, a molecular electronic structure program, Release Dalton 2013 and LSDalton 2013

Aidas, K., Angeli, C., Bak, K., Bakken, V., Bast, R., Boman, L., Christiansen, O., Dahle, P., The DALTON Quantum Chemistry Program System (2014) WIREs Comput. Mol. Sci., 4, pp. 269-284

Voelter, W., Records, R., Bunnenberg, E., Djerassi, C., Magnetic Circular Dichroism Studies. VI. Investigation of Some Purines, Pyrimidines, and Nucleosides (1968) J. Am. Chem. Soc., 90, pp. 6163-6170

Jacquemin, D., Wathelet, V., Perpète, E.A., Adamo, C., Extensive TD-DFT Benchmark: Singlet-Excited States of Organic Molecules (2009) J. Chem. Theory Comput., 5, pp. 2420-2435

Ferrer, F., Cerezo, J., Stendardo, E., Improta, R., Santoro, F., Insights for an Accurate Comparison of Computational Data to Experimental Absorption and Emission Spectra: Beyond the Vertical Transition Approximation (2013) J. Chem. Theory Comput., 9, pp. 2072-2082

Lan, Z., Lu, Y., Fabiano, E., Thiel, W., QM/MM Nonadiabatic Decay Dynamics of 9H-Adenine in Aqueous Solution (2011) ChemPhysChem, 12, pp. 1989-1998

Ludwig, V., Da Costa, Z., Do Amaral, M., Borin, A., Canuto, S., Serrano-Andrés, L., Photophysics and Photostability of Adenine in Aqueous Solution: A Theoretical Study (2010) Chem. Phys. Lett., 492, pp. 164-169

Mennucci, B., Toniolo, A., Tomasi, J., Theoretical Study of the Photophysics of Adenine in Aolution: Tautomerism, Deactivation Mechanisms, and Comparison with the 2-Aminopurine Fluorescent Isomer (2001) J. Phys. Chem. A, 105, pp. 4749-4757

Banyasz, A., Gustavsson, T., Onidas, D., Changenet-Barret, P., Markovitsi, D., Improta, R., Multi-pathway Excited State Relaxation of Adenine Oligomers in Aqueous Solution: A Joint Theoretical and Experimental Study (2013) Chem.-Eur. J., 19, pp. 3762-3774

Santoro, F., Improta, R., Avila, F., Segado, M., Lami, A., The Interplay between Neutral Exciton and Charge Transfer States in Single-Strand Polyadenine: A Quantum Dynamical Investigation (2013) Photochem. Photobiol. Sci., 12, pp. 1527-1543

Olaso-Gonzalez, G., Merchàn, M., Serrano-Andrés, L., The Role of Adenine Excimers in the Photophysics of Oligonucleotides (2009) J. Am. Chem. Soc., 131, pp. 4368-4377

Yamazaki, S., Domcke, W., Sobolewski, A.L., Nonradiative Decay Mechanisms of the Biologically Relevant Tautomer of Guanine (2008) J. Phys. Chem. A, 112, pp. 11965-11968

Fülscher, M., Serrano-Andrés, L., Roos, B., A Theoretical Study of the Electronic Spectra of Adenine and Guanine (1997) J. Am. Chem. Soc., 119, pp. 6168-6176

Sutherland, J.C., Griffin, K., Magnetic Circular Dichroism of Adenine, Hypoxanthine, and Guanosine 5'-Diphosphate to 180 nm (1984) Biopolymers, 23, pp. 2715-2724

Crespo-Hern ndez, C. E., Cohen, B., Hare, P. M., Kohler, B., Ultrafast Excited-State Dynamics in Nucleic Acids (2004) Chem. Rev., 104, pp. 1977-202

Middleton, C. T., De La Harpe, K., Su, C., Law, Y. K., Crespo-Hern ndez, C. E., Kohler, B., DNA Excited-State Dynamics: From Single Bases to the Double Helix (2009) Annu. Rev. Phys. Chem., 60, pp. 217-239

Gonz les, L., Escudero, D., Serrano-Andres, L., Progress and Challenges in the Calculation of Electronic Excited States (2012) ChemPhysChem, 13, pp. 28-51

Serrano-Andr s, L., Merch n, M., Are the Five Natural DNA/RNA Base Monomers a Good Choice from Natural Selection?: A Photochemical Perspective (2009) J. Photochem. Photobiol., C, 10, pp. 21-32

Silva-Junior, M. R., Schreiber, M., Sauer, S. P. A., Thiel, W., Benchmarks for Electronically Excited States: Time-Dependent Density Functional Theory and Density Functional Theory Based Multireference Configuration Interaction (2008) J. Chem. Phys., 129, pp. 104103/1-104103/14

Conti, I., Alto, P., Stenta, M., Garavelli, M., Orlandi, G., Adenine Deactivation in DNA Resolved at the CASPT2//CASSCF/AMBER Level (2010) Phys. Chem. Chem. Phys., 12, pp. 5016-5023

Kwok, W. -M., Ma, C., Phillips, D. L., Femtosecond Time- and Wavelength-Resolved Fluorescence and Absorption Spectroscopic Study of the Excited States of Adenosine and an Adenine Oligomer (2006) J. Am. Chem. Soc., 62, pp. 11894-11905

Mason, W. R., (2007) A Practical Guide to Magnetic Circular Dichroism Spectroscopy, , Wiley: New York

Kj rgaard, T., J rgensen, P., Thorvaldsen, A., Salek, P., Coriani, S., Gauge-Origin Independent Formulation and Implementation of Magneto-Optical Activity within Atomic-Orbital-Density Based Hartree-Fock and Kohn-Sham Response Theories (2009) J. Chem. Theory Comput., 5, pp. 1997-2020

Kj rgaard, T., Kristensen, K., Kauczor, J., J rgensen, P., Coriani, S., Thorvaldsen, A., Comparison of Standard and Damped Response Formulations of Magnetic Circular Dichroism (2011) J. Chem. Phys., 135, p. 024112

Kj rgaard, T., Coriani, S., Ruud, K., Ab Initio Calculation of Magnetic Circular Dichroism (2012) WIREs Comput. Mol. Sci., 2, pp. 443-455

Sutherland, J. C., Holmquist, B., Magnetic Circular Dichroism of Biological Molecules (1980) Annu. Rev. Biophys. Bioeng., 9, pp. 293-326

Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior (1988) Phys. Rev. A, 38, pp. 3098-3100

Dunning, T. H., Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen (1989) J. Chem. Phys., 90, pp. 1007-1023

Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Scalmani, G., Petersson, G. A., (2009) Gaussian 09, , revision C. 3

Yanai, T., Tew, D. P., Handy, N. C., A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP) (2004) Chem. Phys. Lett., 393, pp. 51-57

Peach, M. J. G., Helgaker, T., Sa ek, P., Keal, T. W., Lutnaes, O. B., Tozer, D. J., Handy, N. C., Assessment of a Coulomb-Attenuated Exchange-Correlation Energy Functional (2006) Phys. Chem. Chem. Phys., 8, pp. 558-562

Woon, D. E., Dunning, T. H., Gaussian Basis Sets for Use in Correlated Molecular Calculations. IV. Calculation of Static Electrical Response Properties (1994) J. Chem. Phys., 100, pp. 2975-2988

(2013) DALTON, , http: //daltonprogram. org/, a molecular electronic structure program, Release Dalton 2013 and LSDalton 2013

F lscher, M., Serrano-Andr s, L., Roos, B., A Theoretical Study of the Electronic Spectra of Adenine and Guanine (1997) J. Am. Chem. Soc., 119, pp. 6168-6176

Sutherland, J. C., Griffin, K., Magnetic Circular Dichroism of Adenine, Hypoxanthine, and Guanosine 5'-Diphosphate to 180 nm (1984) Biopolymers, 23, pp. 2715-2724