ХИМИЧЕСКАЯ ФИЗИКА, 2015, том 34, № 1, с. 22-24
СТРОЕНИЕ ХИМИЧЕСКИХ СОЕДИНЕНИЙ, СПЕКТРОСКОПИЯ
THE COMPARISON OF NMR TENSORS AND NQR FREQUENCIES OF HALLUCINOGENIC HARMINE COMPOUND IN THE GAS PHASE
© 2015 Neda Ahmadinejad1, Arezoo Tahan2*
1Young Researchers and Elite Club, Shahre-Rey Branch, Islamic Azad University, Tehran, Iran 2Semnan branch, Islamic Azad University, Semnan, Iran *E-mail: Arezoo.Tahan@gmail.com; A.Tahan@Semnaniau.ac.ir Received 08.04.2014
Density functional theory (DFT) methods were used to analyze the effect of molecular structure and ring currents on the NMR chemical shielding tensors and NQR frequencies of hallucinogenic Harmine in the gas phase. The interpretation of NBO data is represented that in Harmine structure, the lone pair participation of N9 in n-system electron clouds cause to development of aromaticity nature in pyrrole ring. However, the chemical shielding around N9 atom of pyrrole ring has more value of chemical shielding (olso) than N2 atom of pyridine former. It can be deduced that by increasing lone pair electrons contribution of nitrogen atoms in ring resonance interactions and aromaticity development, the values of NMR chemical shielding around them increase while % and qz values of these nuclei decrease.
Keywords: Harmine, NMR shielding tensors, NQR frequencies, NBO analysis.
P- Caroline alkaloids such as Harmine [C13H12N2O] (see Figure) are natural products which are available in a wide range of medicinal plants . They possess different biological natures such as hypotensive, hallucinogenic or antimicrobial activities . Researchers demonstrated that Harmine and other P-Carbolines interfere the action of reactive oxygen species, protecting the nervous system and this behavior is due to their antioxidative properties . The
analysis of P-Carboline alkaloids can be carried out by HPLC with fluorimetric UV-Vis spectrophotometry and mass spectrometry detection [4, 5]. Since Harmine can be a metabolite of Harmane, their separation is very important. However, the application of conventional HPLC techniques to this separation is frequently difficult and micellar electrokinetic chro-matography has been proposed as an alternative. Natural and chemically modified cyclodextrins (CDs) have been profusely used to increase the resolution of
The chemical structure of Harmine.
THE COMPARISON OF NMR TENSORS AND NQR FREQUENCIES 23
Table 1. NMR parameters of 15N nuclei (isotropic, anisotropic, Aoiso, and asymmetric, n, chemical shielding (in ppm) involving in Harmine structure at the B3LYP/6-311++G** level of theory in the gas phase
Structure Nuclei ®iso A®iso n
Harmine N2 n9 -83.9488 131.6290 508.3875 39.8322 0.4933 2.3954
Table 2. Calculated EFG tensors, the NQR parameters and related frequencies of 15N nuclei for Harmine at the B3LYP/6-311++G**//B3LYP/6-31++G** level of theory in the gas phase
Nuclei v_ v0 xzz ne qzz qyy qxx
MHz 1021 V • m_2
n2 4.0514 3.4533 0.5981 5.0031 0.2390 10.1229 -6.2711 -3.8519
n9 3.3184 3.2742 0.4417 4.3950 0.0201 8.8926 -4.5356 -4.3541
chromatographic separations and capillary electrophoresis. On the other hand, CDs provide an enhancement of the sensitivity in the luminescence techniques for the detection of fluorophores [6—8]. In contrast the above mentioned report, there isn't the affective ab initio studies on the interested structures. In this study, with the objective of understanding further the structural facts of hallucinogenic new compounds, we modeled Harmine structure using DFT methods. The basic interest of the present study is to investigate the effects of intra-molecular interactions and ring currents on the NMR chemical shielding and EFG tensors at the site of nitrogen nuclei and finally NBO interpretation of structural factors.
All calculations in present work were performed using the GAUSSIN 03 program . The monomeric structure from crystallographic structure have been optimized at the B3LYP/6-311++ G** level of theory in the gas phase. NBO analysis , the Electric-field gradient (EFG) and GIAO nuclear magnetic shielding calculations were then performed at the same level on the optimized structure. Isotropic chemical shielding (ctSo) was obtained by aiso = (a11 + ct22 + ct33)/3 and Anisotropic chemical shielding (Act) was obtained by Act = ct33 — (ct22 + ct11)/2. The electrostatic interaction of a nuclear electric quadrupole moment and the electron charge cloud surrounding the nucleus can give rise to the observation of pure Nuclear Quadrupole Resonance (NQR) . Quantum chemical calculations yield principal components of the EFG tensor, qu, in atomic units (1 a.u = 9.717365 • 1021 V • m-2) . The calculated qu values were used to obtain the nuclear quadrupole coupling constants:
where Q is the nuclear quadrupole moment of the 15N nuclei. The standard values of quadrupole moment, Q, reported by Pyykko .
Asymmetry parameter is defined as ^q = \(qyy — — qxx)/qzz\, 0 - nQ — 1 that it measures the deviation of the field gradient tensor from axial symmetry. For a nucleus of unit spin (such as 15N), we have three energy levels, so we get three nuclear quadru-pole resonance frequencies :
= 3 X „ (.
3 L n v-=3(1 - 3
■t [MHz] = e2Qqu/h, i = x, y, z,
RESULT AND DISCUSSION
The NMR and NQR parameters of Harmine struc-ture'nitrogens at the B3LYP/6-311++G** level oftheory have also presented in the gas phase (see tables 1 and 2). The obtained results showed that NMR shielding tensors and NQR frequencies are strongly affected by chemical environment and intra-molecular interactions. However, N9 atom of pyrrole ring has more value of chemical shielding (ctSo) than N2 atom of pyridine former. The comparison of NQR parameters of N2 and N9 nuclei is represented that N2 nucleus has more x and qz value than N9 former in the structure. NBO analysis show that the lone pair electrons of pyrrole ring' nitrogen have lower occupancy and higher resonance energy for LP N ^ ct* or n* delocalizations than pyridine ring'nitrogen (see table 3). Based on NBO interpretation and NMR—NQR calculations, it can also be concluded that by increasing lone pair electrons contribution of nitrogen atoms in resonance
XHMH^ECKAÄ OH3HKA TOM 34 № 1 2015
NEDA AHMADINEJAD, AREZOO TAHAN
Table 3. Calculated lone pair occupancies and resonance energies (in kcal • mol for involved atoms and bonds Harmine structure using NBO analysis at the B3LYP/6-311++G** level of theory in the gas phase
Structures Donor NBO Occupancy Charge Acceptor NBO Resonance energy, kcal • mol
BD*(1) Cx-CXQ 9.72
BD*(1) Ci-C14 2.37
LP(1) N2 1.91565 -0.47150 BD*(1) C3-C4 8.94 25.50
Harmine BD*(1) C3-Hn BD*(1) N9 C10 3.94 0.53
LP(1) N9 1.66709 -0.55478 BD*(2) C10-C11 BD*(2) C12 C13 31.93 36.22 68.15
interactions and aromaticity development, the values of NMR chemical shielding around them increase.
This study provides a sensible picture from the in-tra-molecular interactions for Harmine structure. The results represented that the chemical environment and resonance interactions for development of aromaticity nature are effected NMR chemical shielding tensors and NQR parameters on nitrogen nuclei in the gas phase. However, it can be concluded that by increasing lone pair electrons contribution of nitrogen atoms in ring resonance interactions and aromaticity development, the values of NMR chemical shielding around them increase while x and qzz values of these nuclei decrease.
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XHMHTECKAH OH3HKA tom 34 № 1 2015
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