### Abstract

We obtained accurate gas-phase tautomerization energies for a set of 14 guanine tautomers by means of high-level thermochemical procedures approximating the CCSD(T) energy at the complete basis set (CBS) limit. For the five low-lying tautomers, we use the computationally demanding W1-F12 composite method for obtaining the tautomerization energies. The relative W1-F12 tautomerization enthalpies at 298 K are: 0.00 (1), 2.37 (2), 2.63 (3), 4.03 (3′), and 14.31 (4) kJ mol^{-1}. Thus, as many as four tautomers are found within a small energy window of less than 1.0 kcal mol^{-1} (1 kcal mol^{-1} = 4.184 kJ mol^{-1}). We use these highly accurate W1-F12 tautomerization energies to evaluate the performance of a wide range of lower-level composite ab initio procedures. The Gn composite procedures (G4, G4(MP2), G4(MP2)-6X, G3, G3B3, G3(MP2), and G3(MP2)B3) predict that the enol tautomer (3) is more stable than the keto tautomer (2) by amounts ranging from 0.36 (G4) to 1.28 (G3(MP2)) kJ mol^{-1}. We also find that an approximated CCSD(T)/CBS energy calculated as HF/jul-cc-pV{D,T}Z + CCSD/jul-cc-pVTZ + (T)/jul-cc-pVDZ results in a root-mean-square deviation (RMSD) of merely 0.11 kJ mol^{-1} relative to the W1-F12 reference values. We use this approximated CCSD(T)/CBS method to obtain the tautomerization energies of 14 guanine tautomers. The relative tautomerization enthalpies at 298 K are: 0.00 (1), 2.20 (2), 2.51 (3), 4.06 (3′), 14.30 (4), 25.65 (5), 43.78 (4′), 53.50 (6′), 61.58 (6), 77.37 (7), 82.52 (8′), 86.02 (9), 100.70 (10), and 121.01 (8) kJ mol^{-1}. Using these tautomerization enthalpies, we evaluate the performance of standard and composite methods for the entire set of 14 guanine tautomers. The best-performing procedures emerge as (RMSDs are given in parentheses): G4(MP2)-6X (0.51), CCSD(T)+ΔMP2/CBS (0.52), and G4(MP2) (0.64 kJ mol^{-1}). The worst performers are CCSD(T)/AVDZ (1.05), CBS-QB3 (1.24), and CBS-APNO (1.38 kJ mol^{-1}).

Original language | English |
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Pages (from-to) | 607-613 |

Number of pages | 7 |

Journal | Australian Journal of Chemistry |

Volume | 72 |

Issue number | 8 |

DOIs | |

Publication status | Published - 2019 |

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**Thermochemistry of Guanine Tautomers Re-Examined by Means of High-Level CCSD(T) Composite Ab Initio Methods.** / Karton, Amir.

Research output: Contribution to journal › Article

TY - JOUR

T1 - Thermochemistry of Guanine Tautomers Re-Examined by Means of High-Level CCSD(T) Composite Ab Initio Methods

AU - Karton, Amir

PY - 2019

Y1 - 2019

N2 - We obtained accurate gas-phase tautomerization energies for a set of 14 guanine tautomers by means of high-level thermochemical procedures approximating the CCSD(T) energy at the complete basis set (CBS) limit. For the five low-lying tautomers, we use the computationally demanding W1-F12 composite method for obtaining the tautomerization energies. The relative W1-F12 tautomerization enthalpies at 298 K are: 0.00 (1), 2.37 (2), 2.63 (3), 4.03 (3′), and 14.31 (4) kJ mol-1. Thus, as many as four tautomers are found within a small energy window of less than 1.0 kcal mol-1 (1 kcal mol-1 = 4.184 kJ mol-1). We use these highly accurate W1-F12 tautomerization energies to evaluate the performance of a wide range of lower-level composite ab initio procedures. The Gn composite procedures (G4, G4(MP2), G4(MP2)-6X, G3, G3B3, G3(MP2), and G3(MP2)B3) predict that the enol tautomer (3) is more stable than the keto tautomer (2) by amounts ranging from 0.36 (G4) to 1.28 (G3(MP2)) kJ mol-1. We also find that an approximated CCSD(T)/CBS energy calculated as HF/jul-cc-pV{D,T}Z + CCSD/jul-cc-pVTZ + (T)/jul-cc-pVDZ results in a root-mean-square deviation (RMSD) of merely 0.11 kJ mol-1 relative to the W1-F12 reference values. We use this approximated CCSD(T)/CBS method to obtain the tautomerization energies of 14 guanine tautomers. The relative tautomerization enthalpies at 298 K are: 0.00 (1), 2.20 (2), 2.51 (3), 4.06 (3′), 14.30 (4), 25.65 (5), 43.78 (4′), 53.50 (6′), 61.58 (6), 77.37 (7), 82.52 (8′), 86.02 (9), 100.70 (10), and 121.01 (8) kJ mol-1. Using these tautomerization enthalpies, we evaluate the performance of standard and composite methods for the entire set of 14 guanine tautomers. The best-performing procedures emerge as (RMSDs are given in parentheses): G4(MP2)-6X (0.51), CCSD(T)+ΔMP2/CBS (0.52), and G4(MP2) (0.64 kJ mol-1). The worst performers are CCSD(T)/AVDZ (1.05), CBS-QB3 (1.24), and CBS-APNO (1.38 kJ mol-1).

AB - We obtained accurate gas-phase tautomerization energies for a set of 14 guanine tautomers by means of high-level thermochemical procedures approximating the CCSD(T) energy at the complete basis set (CBS) limit. For the five low-lying tautomers, we use the computationally demanding W1-F12 composite method for obtaining the tautomerization energies. The relative W1-F12 tautomerization enthalpies at 298 K are: 0.00 (1), 2.37 (2), 2.63 (3), 4.03 (3′), and 14.31 (4) kJ mol-1. Thus, as many as four tautomers are found within a small energy window of less than 1.0 kcal mol-1 (1 kcal mol-1 = 4.184 kJ mol-1). We use these highly accurate W1-F12 tautomerization energies to evaluate the performance of a wide range of lower-level composite ab initio procedures. The Gn composite procedures (G4, G4(MP2), G4(MP2)-6X, G3, G3B3, G3(MP2), and G3(MP2)B3) predict that the enol tautomer (3) is more stable than the keto tautomer (2) by amounts ranging from 0.36 (G4) to 1.28 (G3(MP2)) kJ mol-1. We also find that an approximated CCSD(T)/CBS energy calculated as HF/jul-cc-pV{D,T}Z + CCSD/jul-cc-pVTZ + (T)/jul-cc-pVDZ results in a root-mean-square deviation (RMSD) of merely 0.11 kJ mol-1 relative to the W1-F12 reference values. We use this approximated CCSD(T)/CBS method to obtain the tautomerization energies of 14 guanine tautomers. The relative tautomerization enthalpies at 298 K are: 0.00 (1), 2.20 (2), 2.51 (3), 4.06 (3′), 14.30 (4), 25.65 (5), 43.78 (4′), 53.50 (6′), 61.58 (6), 77.37 (7), 82.52 (8′), 86.02 (9), 100.70 (10), and 121.01 (8) kJ mol-1. Using these tautomerization enthalpies, we evaluate the performance of standard and composite methods for the entire set of 14 guanine tautomers. The best-performing procedures emerge as (RMSDs are given in parentheses): G4(MP2)-6X (0.51), CCSD(T)+ΔMP2/CBS (0.52), and G4(MP2) (0.64 kJ mol-1). The worst performers are CCSD(T)/AVDZ (1.05), CBS-QB3 (1.24), and CBS-APNO (1.38 kJ mol-1).

UR - http://www.scopus.com/inward/record.url?scp=85069501958&partnerID=8YFLogxK

U2 - 10.1071/CH19276

DO - 10.1071/CH19276

M3 - Article

VL - 72

SP - 607

EP - 613

JO - Australian Journal of Chemistry:an international journal for chemical science

JF - Australian Journal of Chemistry:an international journal for chemical science

SN - 0004-9425

IS - 8

ER -