Search results for "GIAO"

3He NMR studies on helium–pyrrole, helium–indole, and helium–carbazole systems: a new tool for following chemistry of heterocyclic compounds

2015

The 3He nuclear magnetic shieldings were calculated for free helium atom and He–pyrrole, He–indole, and He–carbazole complexes. Several levels of theory, including Hartree–Fock (HF), Second-order Møller-Plesset Perturbation Theory (MP2), and Density Functional Theory (DFT) (VSXC, M062X, APFD, BHandHLYP, and mPW1PW91), combined with polarization-consistent pcS-2 and aug-pcS-2 basis sets were employed. Gauge-including atomic orbital (GIAO) calculated 3He nuclear magnetic shieldings reproduced accurately previously reported theoretical values for helium gas. 3He nuclear magnetic shieldings and energy changes as result of single helium atom approaching to the five-membered ring of pyrrole, indo…

Density functional theory (DFT) prediction of structural and spectroscopic parameters of cytosine using harmonic and anharmonic approximations

2015

The applicability of popular and efficient B3LYP hybrid density functional and medium-size Pople-type basis set in combination with computationally expensive anharmonic model to obtain more accurate theoretical structure, vibrational frequencies and GIAO NMR parameters of cytosine was tested. We report on prediction of cytosine equilibrium (R e ) and rovibrationally averaged (R v ) structures and vibrational frequencies in the gas phase and DMSO solution using density functional theory combined with 6-311++G** basis set. The harmonic and anharmonic vibrational frequencies (using second-order vibrational perturbation theory, VPT2) were critically discussed. In comparison with initial harmoni…

DFT and experimental studies on structure and spectroscopic parameters of 3,6-diiodo-9-ethyl-9H-carbazole

2015

The first report on crystal and molecular structure of 3,6-diiodo-9-ethyl-9H-carbazole is presented. Experimental room-temperature X-ray and 13C chemical shift studies were supported by advanced theoretical calculations using density functional theory (DFT). The 13C nuclear magnetic shieldings were predicted at the non-relativistic and relativistic level of theory using the zeroth-order regular approximation (ZORA). Theoretical relativistic calculations of chemical shifts of carbons C3 and C6, directly bonded to iodine atoms, produced a reasonable agreement with experiment (initial deviation from experiment of 44.3 dropped to 4.25 ppm). The changes in ring aromatic character via simple harm…

Molecular modeling and experimental studies on structure and NMR parameters of 9-benzyl-3,6-diiodo-9H-carbazole

2015

A combined experimental and theoretical study has been performed on 9-benzyl-3,6-diiodo-9H-carbazole. Experimental X-ray (100.0 K) and room-temperature 13C NMR studies were supported by advanced density functional theory (DFT) calculations. The non relativistic structure optimization was performed and the 13C nuclear magnetic shieldings were predicted at the relativistic level of theory using the Zeroth Order Regular Approximation (ZORA). The changes in the benzene and pyrrole rings compared to the unsubstituted carbazole or the parent molecules were discussed in terms of aromaticity changes using the harmonic oscillator model of aromaticity (HOMA) and the nucleus independent chemical shift…

Performance of revised STO(1M)-3G basis set for prediction of 5-fluorocytosine chemical shifts.

2019

Nuclear shieldings and chemical shifts of 5-fluorocytosine (5FC) were predicted in the gas phase and DMSO solution modeled by polarizable continuum model using B3LYP density functional and revised STO(1M)-3G basis set. For comparison, eight arbitrary selected basis sets including STO-3G and medium-size Pople-type and larger dedicated Jensen-type ones were applied. The former basis sets were significantly smaller, but the calculated structural parameters, harmonic vibrational frequencies, were very accurate and close to those obtained with larger, polarization-consistent ones. The predicted 13 C and 1 H chemical shieldings of 5FC and cytosine, selected as parent molecule, were acceptable (ro…

Bioenergian potentiaalitarkastelu Pirkanmaan alueelta - energiaomavaraisuuden jäljillä

2011

Energiaomavaraisuuteen ei Suomessa ole juurikaan kiinnitetty huomiota edullisen tuontienergian aikana. Kuitenkin useat kansainväliset tahot painottavat tarvetta siirtyä kohti energiaomavaraisempaa ja hajautetumpaa energiantuotannon rakennetta. Suomen uusiutuvat energiavarat ovat monipuoliset, ja niiden avulla energiaomavaraisuutta olisi mahdollista parantaa merkittävästi. Tämän pro gradun tarkoituksena oli tarkastella energiaomavaraisuuden tilaa Pirkanmaalta valittujen maaseutuvaltaisten kuntien kautta. Työssä selvitettiin tarkastelualueen tämän hetkinen energiantuotannon infrastruktuuri sekä energiantuotannon ja -kulutuksen rakenteita suhteessa paikallisiin bioenergiavaroihin. Bioenergian …

Digitalisaation harmaat sävyt

2017

3He NMR: from free gas to its encapsulation in fullerene

2013

The 3He nuclear magnetic shieldings were calculated for single helium atom, its dimer, simple models of fullerene cages (He@Cn), and single wall carbon nanotubes. The performances of several levels of theory (HF, MP2, DFT-VSXC, CCSD, CCSD(T), and CCSDT) were tested. Two sets of polarization-consistent basis sets were used (pcS-n and aug-pcS-n), and an estimate of 3He nuclear magnetic shieldings in the complete basis set limit using a two-parameter fit was established. Theoretical 3He results reproduced accurately previously reported theoretical values for helium gas, dimer, and helium probe inside several fullerene cages. Excellent agreement with experimental values was achieved. 3He nuclea…

Comparison of different theory models and basis sets in the calculation of 13C NMR chemical shifts of natural products.

2004

The influence of the calculation method in mimicking experimental 13C NMR chemical shifts of 15 low-polarity natural products singularly containing 10–20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point 13C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the 13C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimi…

Noble Gases as Magnetic Probes in Fullerene Chemistry

2016

Since the early 1990s of the previous century, the use of magnetic active noble gas atom to indirectly probe the size and symmetry of fullerene cage has been reviewed. Molecular modeling of nuclear shielding (and chemical shift) using gauge-including atomic orbital (GIAO) approach at several levels of theory, including Hartree–Fock (HF), density functional theory (DFT), and coupled cluster (CC), is discussed. The advantage of using GIAO calculation to predict and closely reproduce experimental 3He and 129Xe NMR spectra is shown.