NMR quantum computing: applying theoretical methods to designing enhanced systems

Abstract

Density functional theory results for chemical shifts and spin-spin coupling constants are presented for compounds currently used in NMR quantum computing experiments. Specific design criteria were examined and numerical guidelines were assessed. Using a field strength of 7.0 T, protons require a coupling constant of 4 Hz with a chemical shift separation of 0.3 ppm, whereas carbon needs a coupling constant of 25 Hz for a chemical shift difference of 10 ppm, based on the minimal coupling approximation. Using these guidelines, it was determined that 2,3-dibromothiophene is limited to only two qubits; the three qubit system bromotrifluoroethene could be expanded to five qubits and the three qubit system 2,3-dibromopropanoic acid could also be used as a six qubit system. An examination of substituent effects showed that judiciously choosing specific groups could increase the number of available qubits by removing rotational degeneracies in addition to introducing specific conformational preferences that could increase (or decrease) the magnitude of the couplings. The introduction of one site of unsaturation can lead to a marked improvement in spectroscopic properties, even increasing the number of active nuclei.

Unusual long-range spin-spin coupling in fluorinated polyenes: A mechanistic analysis

“…I that active molecules for NMR quantum computing should contain active nuclei with different chemical shieldings. 9 The two F nuclei in 3 are equivalent and do not obey this requirement. By substituting just one of the terminal H atoms in 3 by an electropositive group, this equivalence is alleviated, and one obtains two nonequivalent nuclei while retaining the effective F,F spin-spin coupling observed in 3.…”

“…7,8 The NMR properties of the chemical compound used in this connection are crucial for the performance of the quantum computer. Mawhinney and Schreckenbach 9 have specified the requirements for an appropriate NMR-active molecule to be used for quantum computing: ͑i͒ the molecule should contain as many NMR-active nuclei as possible; ͑ii͒ it should possess different chemical shifts for each active nucleus; ͑iii͒ also needed is a contiguous network of sizable spin-spin coupling constants ͑SSCCs͒ J between the active nuclei. Requirement ͑i͒ implies that the molecules in question should be relatively large.…”

Accurate prediction of195Pt NMR chemical shifts for a series of Pt(ii) and Pt(iv) antitumor agents by a non-relativistic DFT computational protocol

“…The breakthrough in the calculation of the 195 Pt NMR chemical shift can be related to the work of Malkin et al 11 who employed DFT methods with a Kohn-Sham independent gauge for localized orbitals (IGLO). Later, Ziegler and co-workers calculated 17 O NMR shielding tensors in transition metal oxides [MO 4 ] n− and the 195 Pt chemical shift for a series of Pt(II) complexes using a gauge including atomic orbitals (GIAO) and modern DFT methods. Despite the fact that reasonably fast computer codes for 195 Pt NMR computations are now available, 8,12 to accurately predict 195 Pt chemical shifts in solution remains a challenge, due to their sensitivity to the nature of the ligands, to the molecular structure of the complexes, and to the magnitude of solvent effects.…”

1H and 13C NMR Spectra of Natural Products Laurene-Type Sesquiterpenes: Theoretical and Experimental Studies

“…Theoretical calculation of chemical shift has been under development for over three decades 2 and has achieved a high degree of agreement to experimental data. To solve a common difficulty in the calculation of magnetic properties, the gauge origin problem 3 several methods were developed, the most used one is the gauge-including atomic orbital (GIAO) method [4][5][6] .…”