Molecular Magnetic Properties

Transcription

1 Molecular Magnetic Properties Trygve Helgaker Hylleraas Centre, Department of Chemistry, University of Oslo, Norway and Centre for Advanced Study at the Norwegian Academy of Science and Letters, Oslo, Norway European Summer School in Quantum Chemistry (ESQC) 2017 Torre Normanna, Sicily, Italy September 10 23, 2017 Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

2 Sections 1 Introduction 2 The Hamiltonian Zeeman and Hyperfine Interactions 3 First-Order Magnetic Properties 4 Molecular Magnetizabilities 5 High-Resolution NMR Spectra 6 NMR Shielding Constants 7 NMR Spin Spin Coupling Constants Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

3 Introduction Section 1 Introduction Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

4 Introduction Taylor Expansion of Energy Expand the energy in the presence of an external magnetic field B and nuclear magnetic moments M K around zero field and zero moments: E (B, M) = E 0 + perm. magnetic moments {}}{ B T E (10) BT E (20) B }{{} K magnetizability hyperfine coupling {}}{ K B T E (11) K M K } {{ } shieldings + 1 M T K E(01) K + 1 M T K 2 E(02) KL M L + KL }{{} spin spin couplings First-order terms vanish for closed-shell systems because of symmetry they shall be considered only briefly here Second-order terms are important for many molecular properties magnetizabilities nuclear shieldings constants of NMR nuclear spin spin coupling constants of NMR electronic g tensors of EPR (not dealt with here) Higher-order terms are negligible since the perturbations are tiny: 1) the magnetic induction B is weak ( 10 4 a.u.) 2) the nuclear magnetic moments M K couple weakly (µ 0 µ N 10 8 a.u.) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

5 Introduction Derivatives and Perturbation Theory I Consider a normalized CI wave function: c = n=0 cn n, ct c = 1, m n = δ mn The basis functions n are the normalized CI eigenstates of the unperturbed problem: m H n = δ mne n, E 0 E 1 E 2 We here assume that the ground-state energy function depends on two external parameters: c H(x, y) c = cm m H(x, y) n cn, mn n We construct a variational CI Lagrangian: L(x, y, c, µ) = ( c m m H(x, y) n c n µ mn n The stationary conditions are given by c 2 n = 1 ) cn 2 1 L(x, y, c, µ) = 0 c n = 2 n H(x, y) c 2µc n = 0 = H(x, y)c = E 0 (x, y)c L(x, y, c, µ) = 0 µ = cn 2 1 = 0 n = ct c = 1 the first condition is the CI eigenvalue problem with ground-state energy E 0(x, y) = µ the second condition is the CI normalization condition Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

6 Introduction Derivatives and Perturbation Theory II Using the CI Lagrangian, we calculate CI energy derivative in the usual way: de dx = L x, d 2 E dxdy = 2 L x y + n 2 L x c n c n x, 2 L c n c n m c n x = 2 L x c m By inverting the electronic Hessian, we obtain the more compact expression: d 2 E dxdy = 2 L x y 2 [ L 2 ] 1 L 2 L x c mn m c m c n c n y We next evaluate the various partial derivatives at x = y = 0 where c = 0 : L x = 0 H x 0, 2 L x y = 0 2 H x y 0, 2 L c m c n = 2 m H E 0 n = 2(E n E 0 )δ mn 2 L = 2 n H x c x 0 n Inserted above, we recover Rayleigh Schrödinger perturbation theory to second order: de dx = 0 H d 2 E x 0, dxdy = 0 2 H x y H x n n H y 0 E n n E 0 Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

7 The Hamiltonian Zeeman and Hyperfine Interactions Section 2 The Hamiltonian Zeeman and Hyperfine Interactions Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

8 The Hamiltonian Zeeman and Hyperfine Interactions Hamiltonian in Magnetic Field In atomic units, the molecular Hamiltonian is given by H = H 0 + A(r) p + B(r) s + 1 }{{}}{{} 2 A2 (r) }{{} orbital paramagnetic spin paramagnetic diamagnetic There are two kinds of magnetic perturbation operators: paramagnetic (may lower or raise energy) and diamagnetic (always raises energy) There are two kinds of paramagnetic operators: the orbital paramagnetic and spin paramagnetic First- and second-order Rayleigh Schrödinger perturbation theory gives: E (1) = 0 A p + B s 0 E (2) = A 2 0 n 0 A p + B s n n A p + B s 0 E n E 0 In the study of magnetic properties, we are interested in two types of perturbations: externally applied uniform magnetic fields B fields generated internally by nuclear magnetic moments M K Both fields are weak well described by perturbation (response) theory Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

9 The Hamiltonian Zeeman and Hyperfine Interactions Paramagnetic Operators Orbital Paramagnetic Interactions: A p Vector potentials corresponding to uniform fields and nuclear magnetic moments: A O = 1 2 B r O, A K = α 2 M K r K rk 3, α 1/137 the external field is typically about 10 4 a.u. (NMR experiments) the nuclear vector potential is exceedingly small (about 10 8 a.u.) since: α 2 = c a.u., M K = γ K I K 10 4 a.u. We obtain the following orbital paramagnetic operators: A O p = 1 2 B r O p = A K p = α 2 M K r K p r 3 K 1 2 B r O p = = α 2 M K r K p r 3 K 1 2 B L O orbital Zeeman = α 2 M K LK r 3 K orbital hyperfine interactions depend on angular momenta L O and L K relative to O and R K, respectively orbital hyperfine interaction expressed in terms of the paramagnetic spin orbit operator: A K p = M K h PSO K, hpso K = α 2 L K rk 3 These are imaginary singlet operators they have zero expectation values of closed-shell states they generate complex wave functions Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

10 The Hamiltonian Zeeman and Hyperfine Interactions Paramagnetic Operators Spin Paramagnetic Interactions: B s The spin interaction with the external uniform field B is trivial: B s spin Zeeman interaction should be compared with the orbital Zeeman interaction 1 2 B L O (different prefactor!) Taking the curl of A K, we obtain the nuclear magnetic field: B K = A K = 8πα2 3 δ(r K )M K + α 2 3r K (r K M K ) rk 2 M K rk 5 the first term is a contact interaction and contributes only at the nucleus the second term is a classical dipole field and contributes at a distance This magnetic field B K thus gives rise to two spin hyperfine interactions: B K s = M K (h FC K + hsd K ), h FC K h SD = 8πα2 3 δ(r K ) s Fermi contact (FC) K = α2 3r K rk T r K 2 I 3 r K 5 s spin dipole (SD) the FC operator contributes when the electron passes through the nucleus the SD operator is a classical dipolar interaction, decaying as r 3 K These are real triplet operators, which change the spin of the wave function they have zero expectation values of closed-shell states they couple closed-shell states to triplet states Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

11 The Hamiltonian Zeeman and Hyperfine Interactions Hamiltonian with Zeeman and Hyperfine Operators Perturbation Theory with Zeeman and Hyperfine Operators Hamiltonian with a uniform external field and with nuclear magnetic moments: H = H 0 + H (1) + H (2) = H 0 + H z (1) + H (1) hf A2 Zeeman interactions with the external magnetic field B: H (1) z = 1 2 B L O + B s 10 4 hyperfine interactions with the nuclear magnetic moments M K : H (1) hf = K M K h PSO K + K ( M K h FC K ) + hsd K 10 8 h PSO K = α 2 L K rk 3, h FC K = 8πα2 3 δ(r K ) s, h SD K = 3r α2 K rk T r K 2 I 3 rk 5 s Second-order Rayleigh Schrödinger perturbation theory: E (1) = 0 (1) H z + H (1) hf 0 E (2) = 1 0 A H (1) z + H (1) hf n n H (1) z + H (1) hf 0 2 E n n E 0 Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

12 The Hamiltonian Zeeman and Hyperfine Interactions Hamiltonian with Zeeman and Hyperfine Operators Zeeman and Hyperfine Interactions Zeeman SO SS, SO, OO SO PSO FC+SD hyperfine FC+SD PSO Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

13 The Hamiltonian Zeeman and Hyperfine Interactions Diamagnetic Operators Diamagnetic Operators: 1 2 A2 From A O and A K, we obtain three diamagnetic operators: A = A O + A K = A 2 = A O A O + 2A O A K + A K A K Their explicit forms and typical magnitudes (atomic units) are given by A O A O = 1 4 (B r O) (B r O ) 10 8 A O A K = α2 2 These are all real singlet operators (B r O ) (M K r K ) r 3 K A K A L = α 4 (M K r K ) (M L r L ) r 3 K r 3 L their expectation values contribute to second-order magnetic properties they are all exceedingly small but nonetheless all observable Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

14 First-Order Magnetic Properties Section 3 First-Order Magnetic Properties Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

15 First-Order Magnetic Properties Taylor Expansion of Energy Expand the energy in the presence of an external magnetic field B and nuclear magnetic moments M K around zero field and zero moments: E (B, M) = E 0 + perm. magnetic moments {}}{ B T E (10) BT E (20) B }{{} K magnetizability hyperfine coupling {}}{ K B T E (11) K M K } {{ } shieldings + 1 M T K E(01) K + 1 M T K 2 E(02) KL M L + KL }{{} spin spin couplings First-order terms vanish for closed-shell systems because of symmetry they shall be considered only briefly here Second-order terms are important for many molecular properties magnetizabilities nuclear shieldings constants of NMR nuclear spin spin coupling constants of NMR electronic g tensors of EPR (not dealt with here) Higher-order terms are negligible since the perturbations are tiny: 1) the magnetic induction B is weak ( 10 4 a.u.) 2) the nuclear magnetic moments M K couple weakly (µ 0 µ N 10 8 a.u.) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

16 First-Order Magnetic Properties First-Order Magnetic Properties First-Order Molecular Properties The first-order properties are expectation values of H (1) Permanent magnetic moment M = 0 (1) H z 0 = L O + s 0 permanent magnetic moment dominates the magnetism of molecules the molecule reorients itself and enters the field such molecules are therefore paramagnetic Hyperfine coupling constants A K = 0 (1) H hf 0 = 8πα δ (rk ) s 0 MK + measure spin density at the nucleus important in electron paramagnetic resonance (EPR) recall: there are three hyperfine mechanisms: FC, SD and PSO Note: there are no first-order Zeeman or hyperfine couplings for closed-shell molecules all expectation values vanish for imaginary operators and triplet operators: c.c. ˆΩ imaginary c.c. c.c. ˆΩ triplet c.c. 0 Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

17 Molecular Magnetizabilities Section 4 Molecular Magnetizabilities Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

18 Molecular Magnetizabilities Molecular Magnetizabilities Expand the molecular electronic energy in the external magnetic field: E (B) = E 0 B T M 1 2 BT ξb + The magnetizability describes the second-order energy: 0.1 ξ = d2 E db 2 = 0 2 H B H B n n H 0.08 B E n n E 0 = ro T 4 r OI 3 r O ro T LO n n L T O E }{{} n n E 0 } {{ } 0.1 diamagnetic term paramagnetic term The magnetizability describes the curvature at zero magnetic field: a) a) c) b) x b) d) x left: diamagnetic dependence on the field (ξ < 0); right: paramagnetic dependence on the field (ξ > 0) c) d) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

19 Molecular Magnetizabilities Zeeman and Hyperfine Interactions Zeeman SO SS, SO, OO SO PSO FC+SD hyperfine FC+SD PSO Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

20 Molecular Magnetizabilities Basis-Set Convergence of Hartree Fock Magnetizabilities London orbitals are correct to first-order in the external magnetic field For this reason, basis-set convergence is usually improved RHF magnetizabilities of benzene: basis set χ xx χ yy χ zz London STO-3G G cc-pvdz aug-cc-pvdz origin CM STO-3G G cc-pvdz aug-cc-pvdz origin H STO-3G G cc-pvdz aug-cc-pvdz Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

21 Molecular Magnetizabilities Normal Distributions of Errors for Magnetizabilities Normal distributions of magnetizability errors for 27 molecules in the aug-cc-pcvqz basis relative to CCSD(T)/aug-cc-pCV[TQ]Z values (Lutnæs et al., JCP 131, (2009)) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

22 Molecular Magnetizabilities Mean Absolute Errors for Magnetizabilities Mean relative errors (MREs, %) in magnetizabilities of 27 molecules relative to the CCSD(T)/aug-cc-pCV[TQ]Z values. The DFT results are grouped by functional type. The heights of the bars correspond to the largest MRE in each category. (Lutnæs et al., JCP 131, (2009)) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

23 High-Resolution NMR Spectra Section 5 High-Resolution NMR Spectra Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

24 High-Resolution NMR Spectra NMR Spin Hamiltonian High-Resolution NMR Spin Hamiltonian Consider a molecule in an external magnetic field B along the z axis and with nuclear spins I K related to the nuclear magnetic moments M K as: M K = γ K I K 10 4 a.u. where γ K is the magnetogyric ratio of the nucleus. Assuming free molecular rotation, the nuclear magnetic energy levels can be reproduced by the following high-resolution NMR spin Hamiltonian: where we have introduced H NMR = γ K (1 σ K )BI K z + γ K γ L 2 K KL I K I L K K>L }{{}}{{} nuclear Zeeman interaction nuclear spin spin interaction the nuclear shielding constants σ K the (reduced) indirect nuclear spin spin coupling constants K KL This is an effective nuclear spin Hamiltonian: it reproduces NMR spectra without considering the electrons explicitly the spin parameters σ K and K KL are adjusted to fit the observed spectra we shall consider their evaluation from molecular electronic-structure theory Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

25 High-Resolution NMR Spectra NMR Spin Hamiltonian Simulated 200 MHz NMR spectra of Vinyllithium 12 C 2 H 3 6 Li Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

26 NMR Shielding Constants Section 6 NMR Shielding Constants Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

27 NMR Shielding Constants Nuclear Shielding Constants Nuclear Shielding Constants Expansion of closed-shell energy in an external field B and nuclear magnetic moments M K : Here E (11) K E (B, M) = E BT E (20) B K B T E (11) K M K KL M T K E(02) KL M L + describes the coupling between the applied field and the nuclear moments: in the absence of electrons (i.e., in vacuum), this coupling is identical to I 3 : Hz nuc = B M K the purely nuclear Zeeman interaction K in the presence of electrons (i.e., in a molecule), the coupling is modified slightly: E (11) K = I 3 + σ K the nuclear shielding tensor Shielding constants arise from a hyperfine interaction between the electrons and the nuclei and Zeeman interaction between the electrons and the field they are of the order of α and are measured in ppm The nuclear Zeeman interaction does not enter the electronic problem compare with the nuclear nuclear Coulomb repulsion Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

28 NMR Shielding Constants Nuclear Shielding Constants Zeeman and Hyperfine Interactions Zeeman SO SS, SO, OO SO PSO FC+SD hyperfine FC+SD PSO Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

29 NMR Shielding Constants Nuclear Shielding Constants Ramsey s Expression for Nuclear Shielding Tensors Ramsey s expression for nuclear shielding tensors of a closed-shell system: σ K = d2 E el = 0 2 H dbdm K B M H B n n H M 0 K K E n n E 0 = α2 ro T 0 r K I 3 r O rk T 2 rk 3 0 α 0 LO n n r 3 2 K LT K 0 E n n E 0 }{{}}{{} diamagnetic term paramagnetic term The (usually) dominant diamagnetic term arises from differentiation of the operator: A O A K = 1 2 α2 r 3 K (B r O) (M K r K ) As for the magnetizability, there is no spin contribution for singlet states: S 0 0 singlet state For 1 S systems (closed-shell atoms), the paramagnetic term vanishes completely and the shielding is given by (assuming gauge origin at the nucleus): σ Lamb = α2 S r 1 K 1 S Lamb formula Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

30 NMR Shielding Constants Nuclear Shielding Constants Benchmark Calculations of BH Shieldings (ppm) σ( 11 B) σ( 11 B) σ( 1 H) σ( 1 H) HF MP CCSD CCSD(T) CCSDT CCSDTQ CISD CISDT CISDTQ FCI TZP+ basis, R BH = pm, all electrons correlated J. Gauss and K. Ruud, Int. J. Quantum Chem. S29 (1995) 437 M. Kállay and J. Gauss, J. Chem. Phys. 120 (2004) 6841 Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

31 NMR Shielding Constants Nuclear Shielding Constants Coupled-Cluster Convergence of Shielding Constants in CO (ppm) CCSD CCSD(T) CCSDT CCSDTQ CCSDTQ5 FCI σ( 13 C) σ( 13 C) σ( 17 O) σ( 17 O) All calculations in the cc-pvdz basis and with a frozen core. Kállay and Gauss, J. Chem. Phys. 120 (2004) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

32 NMR Shielding Constants Nuclear Shielding Constants Calculated and Experimental Shielding Constants (ppm) HF CAS MP2 CCSD CCSD(T) exp. HF F ± 6 (300K) H ± 0.2 (300K) H 2O O ± 6 (300K) H ± 0.02 NH 3 N H ± 1.0 CH 4 C H F 2 F N 2 N ± 0.2 (300K) CO C ± 0.9 (eq) O ± 6 (eq) For references and details, see Chem. Rev. 99 (1999) 293. for exp. CO and H 2 O values, see Wasylishen and Bryce, JCP 117 (2002) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

33 NMR Shielding Constants Nuclear Shielding Constants Kohn Sham shielding constants (ppm) HF LDA BLYP B3LYP KT2 CCSD(T) exp. HF F ± 6 H 2O O ± 6 NH 3 N CH 4 C F 2 F N 2 N ± 0.2 CO C ± 0.9 (eq) O ± 6 (eq) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

34 NMR Shielding Constants Nuclear Shielding Constants MAE (Exp.) MAE (Emp. Eq.) NMR: Mean Absolute Errors Relative to Experiment Mean absolute errors relative to experimental (blue) and empirical equilibrium values (red) MAE / ppm MAE (Exp.) MAE (Emp. Eq.) 0 Kohn Sham calculations give shielding constants of uneven quality errors increase when vibrational corrections are applied Teale et al. JCP 138, (2013) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

35 NMR Shielding Constants Nuclear Shielding Constants Mean absolute NMR shielding errors relative to empirical equilibrium values RHF LDA MAE / ppm CC GGA Hybrid OEP-Hybrid H C N O F All 10 0 Mean absolute errors (in ppm) for NMR shielding constants relative to empirical equilibrium values for H (white), C (grey), N (blue), O (red), and F (yellow). The total mean absolute errors over all nuclear types are shown by the purple bars. The DFT methodologies are arranged in the categories LDA, GGA, hybrid and OEP-hybrid. (Teal et al., JCP) Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

36 NMR Spin Spin Coupling Constants Section 7 NMR Spin Spin Coupling Constants Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

37 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Direct and Indirect Nuclear Spin Spin Couplings The last term in the expansion of the molecular electronic energy in B and M K E (B, M) = E BT E (20) B K BT E (11) K M K KL MT K E(02) KL M L + describes the coupling of the nuclear magnetic moments in the presence of electrons There are two distinct contributions to the coupling: the direct and indirect contributions E (02) KL = D KL + K KL The direct coupling occurs by a classical dipole mechanism: D KL = α 2 R 5 ( KL R 2 KL I 3 3R KL R T ) KL a.u. it is anisotropic and vanishes in isotropic media such as gases and liquids The indirect coupling arises from hyperfine interactions with the surrounding electrons: it is exceedingly small: K KL a.u. 1 Hz it does not vanish in isotropic media it gives the fine structure of high-resolution NMR spectra Experimentalists usually work in terms of the (nonreduced) spin spin couplings J KL = h γ K 2π γ L 2π K KL isotope dependent Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

38 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Zeeman and Hyperfine Interactions Zeeman SO SS, SO, OO SO PSO FC+SD hyperfine FC+SD PSO Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

39 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Ramsey s Expression for Indirect Nuclear Spin Spin Coupling Tensors The indirect nuclear spin spin coupling tensors of a closed-shell system are given by: K KL = d2 E el = 0 2 H dm K dm L M K M H M n n H K M 0 L L E n n E 0 Carrying out the differentiation of the Hamiltonian, we obtain Ramsey s expression: K KL = α 0 4 rk T r LI 3 r K rl T rk 3 r L 3 0 2α 0 r 3 4 K L K n n r 3 L LT L 0 E n n E 0 }{{}}{{} diamagnetic spin orbit (DSO) paramagnetic spin orbit (PSO) 0 8π 3 δ(r K )s + 3r K rk T r K 2 I 3 s n n 8π 3 δ(r L)s T + 3r LrL T r L 2 I 3 s T 0 r 5 K r 5 L 2α 4 E n n E 0 }{{} Fermi contact (FC) and spin dipole (SD) the isotropic FC/FC term often dominates short-range coupling constants the FC/SD and SD/FC terms often dominate the anisotropic part of K KL the orbital contributions (especially DSO) are usually but not invariably small for large internuclear separations, the DSO and PSO contributions cancel Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

40 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Relative Importance of Contributions to Spin Spin Coupling Constants The isotropic indirect spin spin coupling constants can be uniquely decomposed as: J KL = J DSO KL + J PSO KL + JFC KL + JSD KL The spin spin coupling constants are often dominated by the FC term Since the FC term is relatively easy to calculate, it is tempting to ignore the other terms. However, none of the contributions can be a priori neglected (N 2 and CO)! 200 PSO FC PSO FC FC FC FC FC SD FC FC PSO FC SD FC -100 SD H2 HF H2O O H NH3 N H CH4 C2H4 HCN C H C C N C N2 CO C2H2 C C Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

41 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Calculation of Indirect Nuclear Spin Spin Coupling Constants The calculation of spin spin coupling constants is a challenging task Spin spin coupling constants depend on many coupling mechanisms: 3 singlet response equations and 7 triplet equations for each nucleus for shieldings, only 3 equations are required, for molecules of all sizes Spin spin coupling constants require a proper description of static correlation the Hartree Fock model fails abysmally MCSCF theory treats static correlation properly but is expensive Spin spin couplings are sensitive to the basis set the FC contribution requires an accurate electron density at the nuclei steep s functions must be included in the basis Spin spin couplings are sensitive to the molecular geometry equilibrium structures must be chosen carefully large vibrational corrections (often 5% 10%) For heavy elements, a relativistic treatment may be necessary. However, there is no need for London orbitals since no external magnetic field is involved. Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

42 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Restricted Hartree Fock Theory and Triplet Instabilities The correct description of triplet excitations is important for spin spin coupling constants In restricted Hartree Fock (RHF) theory, triplet excitations are often poorly described upon H 2 dissociation, RHF does not describe the singlet ground state correctly but the lowest triplet state dissociates correctly, leading to triplet instabilities 1 D 2p g FCI 1 P 1s2p 3 P 1s2p 1 1 g 1Σ u 2 1 u 1Σg1Σu R ionic cov ion 3 u 1Σg1Σu covalent 2 1 g 1Σ g 2 1 S 1s 2 g FCI Near such instabilities, the RHF description of spin interactions becomes unphysical C 2H 4/Hz 1 J CC 1 J CH 2 J CH 2 J HH 3 J cis 3 J trans exp RHF CAS B3LYP Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

43 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Reduced Spin Spin Coupling Constants by Wave-Function Theory RHF CAS RAS SOPPA CCSD CC3 exp vib HF 1 K HF CO 1 K CO N 2 1 K NN H 2O 1 K OH K HH NH 3 1 K NH K HH C 2H 4 1 K CC K CH K CH K HH K cis K tns abs at R e % SOPPA: second-order polarization-propagator approximation Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

44 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Reduced spin spin coupling constants by density-functional theory LDA BLYP B3LYP PBE B97-3 RAS exp vib HF 1 K HF CO 1 K CO N 2 1 K NN H 2O 1 K OH K HH NH 3 1 K NH K HH C 2H 4 1 K CC K CH K CH K HH K cis K tns abs at R e % Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

45 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Comparison of Density-Functional and Wave-Function Theory Normal distributions of errors for indirect nuclear spin spin coupling constants for the same molecules as on the previous slides HF LDA BLYP B3LYP CAS RAS SOPPA CCSD Some observations: LDA underestimates only slightly, but has a large standard deviation BLYP reduces the LDA errors by a factor of two B3LYP errors are similar to those of CASSCF The CCSD method is slightly better than the SOPPA method Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

46 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants The Karplus curve Vicinal (three-bond) spin spin coupling constants depend critically on the dihedral angle: 3 J HH in ethane as a function of the dihedral angle: empirical DFT Good agreement with the (empirically constructed) Karplus curve Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

47 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Valinomycin C 54 H 90 N 8 O 18 DFT can be applied to large molecular systems such as valinomycin (168 atoms) there are a total of 7587 spin spin couplings to the carbon atoms in valinomycin below, we have plotted the magnitude of the reduced LDA/6-31G coupling constants on a logarithmic scale, as a function of the internuclear distance: the coupling constants decay in characteristic fashion most of the indirect couplings beyond 500 pm are small and cannot be detected Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

48 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Valinomycin C 54 H 90 N 8 O 18 One-bond spin spin couplings to CH, CO, CN, CC greater than 0.01 Hz Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

49 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Valinomycin C 54 H 90 N 8 O 18 Two-bond spin spin couplings to CH, CO, CN, CC greater than 0.01 Hz Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

50 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Valinomycin C 54 H 90 N 8 O 18 Three-bond spin spin couplings to CH, CO, CN, CC greater than 0.01 Hz Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51

51 NMR Spin Spin Coupling Constants Indirect Nuclear Spin Spin Coupling Constants Valinomycin C 54 H 90 N 8 O 18 Four-bond spin spin couplings to CH, CO, CN, CC greater than 0.01 Hz Trygve Helgaker (University of Oslo) Molecular Magnetic Properties ESQC / 51