## Abstract

In this work we report the results of a combined matrix-isolation FT-IR and theoretical computational DFT and ab initio study of the tautomeric and vibrational characteristics of 1,7-dimethyl-guanine (17DMG). The HF, MP2 and DFT methods are all in agreement in predicting that the amino-oxo tautomer of 17DMG is the most stable form, and that the imino-oxo tautomer, which can exist in two different geometrical isomers, is the second most stable form. The energy difference between the amino-oxo and amino-oxo tautomer is rather large: 23, 25 and 33 kJ mol^{-1}, for the HF, DFT and MP2 methods, respectively. The imino-hydroxy tautomer is, according to the calculations, by far the least stable form (ΔE > 180 kJ mol^{-1}). The FT-IR spectra of 17DMG isolated in an argon matrix are in agreement with the theoretical calculation indicating that only the amino-oxo tautomer should be present in the gas phase in detectable amounts. The experimental IR frequencies agree quite well with the theoretical values. When a single scaling factor is applied to approximately correct the calculated frequencies for various systematic errors in theoretical approach, the mean frequency deviations for the HF/6-31++G**, DFT /6-31G** and DFT/6-31++G** calculated spectra are 17.5 cm^{-1}, 12.2 cm^{-1} and 16.3 cm^{-1}, respectively. The DFT mean-frequency-deviations decrease to 8.1 cm^{-1} and 10.1 cm^{-1} when variable factors are applied. The stabilities of the different water complexes of the amino-oxo tautomer of 17DMG are investigated by optimizing the complexes at the HF/6-31++G** level of theory and calculating the MP2 energies at these optimized geometries. The N_{3}...HO-H complex, which has a close structure with two H-bonds, is predicted to be the most stable structure. The MP2 calculated energy differences between the most stable complex and the C_{6}=O...HO-H, the H-NH...OH_{2} and the H_{2}N...HO-H hetero-dimers are 14.28 kJ mol^{-1}, 1954 kJ mol^{-1} and 20.73 kJ mol^{-1}, respectively.

Original language | English (US) |
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Pages (from-to) | 201-220 |

Number of pages | 20 |

Journal | Journal of Molecular Structure |

Volume | 442 |

Issue number | 1-3 |

DOIs | |

State | Published - Feb 3 1998 |

## Keywords

- Ab initio calculation
- Density functional theory
- Hydrogen bonding
- Tautomerism
- Vibration spectrum

## ASJC Scopus subject areas

- Analytical Chemistry
- Spectroscopy
- Organic Chemistry
- Inorganic Chemistry

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