Locating Cytosine Conical Intersections by Laser Experiments andAb InitioCalculations

Abstract

The decay mechanism of S 0 → S 1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250−1200 cm −1 above the 0 0 0 bands. The break-off in the spectra correlates with the calculated barriers toward the "C5−C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S 1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following ( 1 ππ*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm −1 .

Ultrafast excited‐state dynamics of gas‐phase 5‐methylcytosine tautomers

“…[3][4][5][6] Among all nucleobases, cytosine (Cy) and its derivatives are probably the most studied system experimentally in the gas phase. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Ultrafast excited-state dynamics of Cy in the gas phase has been explored by several groups. [22][23][24][25][26] Our group has previously used femtosecond pump-probe photoionization spectroscopy to investigate the ultrafast excited-state dynamics of Cy and its microsolvated complexes.…”

Quantum dynamics of the ππ*/nπ* decay of the epigenetic nucleobase 1,5-dimethyl-cytosine in the gas phase

“…5,6 In particular it is more prone than cytosine (Cyt), its parent base, to undergo photodimerization. [7][8][9][10] The photoactivated dynamics of 5Me-Cyt and 5-methyl(deoxy)cytidine (5Me-dCyd) nucleoside present in DNA has thus been rather extensively studied by experiments 9,[11][12][13][14][15][16] and calculations. 10,13,[16][17][18][19][20][21] Methyl substitution at position 5 has been shown to increase the excited state lifetime of Cyt by almost one order of magnitude, i.e.…”

“…11,12,14,15,18 A similar trend is found also for jet-cooled molecules in the gas phase. 16 This result has been related to the non-radiative decay path of the lowest energy bright transition, which is a HOMO→LUMO ππ * excitation for both for Cyt, 5Me-Cyt and 1,5diMe-Cyt, often used as a model for 5Me-dCyd. 16,18 It involves the so-called C5-C6 ethylenic Conical Intersection (CoI), characterized by the motion of the C5 and C6 substituent out from the molecular plane; the associated energy barrier, very small for Cyt, is increased by 5-methylation.…”

“…16 This result has been related to the non-radiative decay path of the lowest energy bright transition, which is a HOMO→LUMO ππ * excitation for both for Cyt, 5Me-Cyt and 1,5diMe-Cyt, often used as a model for 5Me-dCyd. 16,18 It involves the so-called C5-C6 ethylenic Conical Intersection (CoI), characterized by the motion of the C5 and C6 substituent out from the molecular plane; the associated energy barrier, very small for Cyt, is increased by 5-methylation. 16,18 A methyl in position 5 also suppresses a deactivation channel, 14,15 with a characteristic time of ∼30 ps, identified for Cyt and (deoxy)Cytidine (dCyd) in water 14,15 and in chloroform, 22 and assigned to a dark nπ * transition.…”

“…16,18 It involves the so-called C5-C6 ethylenic Conical Intersection (CoI), characterized by the motion of the C5 and C6 substituent out from the molecular plane; the associated energy barrier, very small for Cyt, is increased by 5-methylation. 16,18 A methyl in position 5 also suppresses a deactivation channel, 14,15 with a characteristic time of ∼30 ps, identified for Cyt and (deoxy)Cytidine (dCyd) in water 14,15 and in chloroform, 22 and assigned to a dark nπ * transition. 18 Actually, different computa-tional models predict that, for both Cyt and 5Me-Cyt, two 'dark' nπ * states, involving the Lone Pair of N3 and O7, fall in the energy range of the two lowest energy ππ * transitions.…”