A HF/CI-SD study of the low-lying states of nitroprusside ion

Journal of the Brazilian Chemical Society

Hollauer, Eduardo¹  Olabe, José A.¹  Universidade Federal Fluminense, Niterói, Brazil¹  Universidad de Buenos Aires, Ciudad Universitaria, Argentina¹ 

关键词: ab initio (SCF;    CI-SD);    nitroprusside;    excited states;    metastable states;    Na2Fe(CN)5NO;    pentacyanonitrosylferrate    Introduction The disodium pentacyanonitrosylferrate (sodium nitroprusside);    Na2Fe(CN)5NO.2H2O (SNP) is a dark red crystal which structure is well known to be ortorhombic Pnnm with four molecules per unit cell1. The Fe(CN)5NO-2 anion shows an approximate 4m local symmetry and the octahedra structure slightly distorts in the crystal with the angle NC-Fe-NO departing from linearity by about 3°. This complex shows a rather unusual Fe-NO distance;    1.653 Å;    and a slightly extended N-O distance;    1.124 Å;    typical of a strong NO ® metals electron donation followed by intense metal ® NO back-donation. Another point of interest are the Fe-CN distances that have nearly the same value for the equatorial (1.933 Å;    averaged;    cis hereafter) and axial ligands (1.918 Å;    trans hereafter). For complexes of the type M(CN)5NO-2;    where M is a metal of group VIII;    the trans Fe-CN distance is usually longer than cis distances. The trans Fe-CN is nearly linear with the Fe-NO bond but two possible orientations are found for the Fe-NO bond in the crystal. They form an angle of 112°10 in an almost anti-parallel orientation;    both ±33°55 apart from a axis. Its UV-visible spectrum does not change qualitatively in solution supporting that environmental effects do not break the qualitative picture obtained from calculations on the isolated anion2. Recently interest in elucidating the electronic structure of SNP has been reinforced by the discovery of similar long-living metastable states and to the increasing biological importance of this ion;    kindly pointed by the referees. Following their suggestion is important to remark that nitroprusside is a drug;    being presently commercialized in the USA under the name of Nipride;    and takes important hole activating guanylyl cyclase;    either via release of nitric oxide or via direct stimulation of the enzyme. The result is increased intracellular cBMP;    which relaxes vascular smooth muscle by decreasing calcium levels in the cell. Nitroprusside acts as a vasodilator;    decreases blood pressure and increases cardiac output. However;    metabolism of nitroprusside results in the cyanide release;    a very serious side effect. As such;    nitroprusside is used parentally in treating hypertensive emergencies and severe cardiac failure.Early calculations on the electronic structure of SNP have been carried out by Manoharan and Gray2 at SCCC-EHT level. The ten highest occupied molecular orbitals (HOMO) obtained were (core) 8e4 5b12 6b12 9e4 21a12 1a22 10e4 11e4 12e4 2b22with the 2b2 showing a major contribution of 3d(xy) metal orbitals and the 13e appearing as a mixture of 3d(xz;    yz) with the dominant contribution of p* NO. The average energy difference between the ten highest orbitals;    is around 0.5 eV with the two HOMOs lying within 0.56 eV;    and apart by 2.3 eV from the others in Koopmans energy. Among the first virtual orbitals (LUMO) we find;    in increasing energy order;    13e 7b1 22a1 14e 23a1;    well separated in energy (@ 4 eV) and with a HOMO-LUMO gap of around 2.54 eV. This pattern clearly indicates a spectrum dominantly governed by one-electron excitations from the two HOMOs to the first LUMOs. Based on this pattern Manoharan and Gray proposed the transition assignment presented inTable 1;    for which the low energy excitations are clearly low-intensity CT transitions. These results were confirmed by Fenske and DeKock3 with a modified CNDO method and other experimental works on the optical electronic spectrum;    in solution and with polarized light in crystals4;    ESR experiments3;    XPS experiments5 and by Mossbauer6 spectroscopy claim these assignments to be correct. ;   

DOI  :  10.1590/S0103-50531997000500010

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The disodium pentacyanonitrosylferrate (sodium nitroprusside), Na2Fe(CN)5NO.2H2O (SNP) is a dark red crystal which structure is well known to be ortorhombic Pnnm with four molecules per unit cell1. The Fe(CN)5NO-2 anion shows an approximate 4m local symmetry and the octahedra structure slightly distorts in the crystal with the angle NC-Fe-NO departing from linearity by about 3°. This complex shows a rather unusual Fe-NO distance, 1.653 Å, and a slightly extended N-O distance, 1.124 Å, typical of a strong NO ® metals electron donation followed by intense metal ® NO back-donation. Another point of interest are the Fe-CN distances that have nearly the same value for the equatorial (1.933 Å; averaged; cis hereafter) and axial ligands (1.918 Å; trans hereafter). For complexes of the type M(CN)5NO-2, where M is a metal of group VIII, the trans Fe-CN distance is usually longer than cis distances. The trans Fe-CN is nearly linear with the Fe-NO bond but two possible orientations are found for the Fe-NO bond in the crystal. They form an angle of 112°10 in an almost anti-parallel orientation, both ±33°55 apart from a axis. Its UV-visible spectrum does not change qualitatively in solution supporting that environmental effects do not break the qualitative picture obtained from calculations on the isolated anion2. Recently interest in elucidating the electronic structure of SNP has been reinforced by the discovery of similar long-living metastable states and to the increasing biological importance of this ion, kindly pointed by the referees. Following their suggestion is important to remark that nitroprusside is a drug, being presently commercialized in the USA under the name of Nipride, and takes important hole activating guanylyl cyclase, either via release of nitric oxide or via direct stimulation of the enzyme. The result is increased intracellular cBMP, which relaxes vascular smooth muscle by decreasing calcium levels in the cell. Nitroprusside acts as a vasodilator, decreases blood pressure and increases cardiac output. However, metabolism of nitroprusside results in the cyanide release, a very serious side effect. As such, nitroprusside is used parentally in treating hypertensive emergencies and severe cardiac failure.Early calculations on the electronic structure of SNP have been carried out by Manoharan and Gray2 at SCCC-EHT level. The ten highest occupied molecular orbitals (HOMO) obtained were (core) 8e4 5b12 6b12 9e4 21a12 1a22 10e4 11e4 12e4 2b22with the 2b2 showing a major contribution of 3d(xy) metal orbitals and the 13e appearing as a mixture of 3d(xz,yz) with the dominant contribution of p* NO. The average energy difference between the ten highest orbitals, is around 0.5 eV with the two HOMOs lying within 0.56 eV, and apart by 2.3 eV from the others in Koopmans energy. Among the first virtual orbitals (LUMO) we find, in increasing energy order, 13e 7b1 22a1 14e 23a1 , well separated in energy (@ 4 eV) and with a HOMO-LUMO gap of around 2.54 eV. This pattern clearly indicates a spectrum dominantly governed by one-electron excitations from the two HOMOs to the first LUMOs. Based on this pattern Manoharan and Gray proposed the transition assignment presented inTable 1, for which the low energy excitations are clearly low-intensity CT transitions. These results were confirmed by Fenske and DeKock3 with a modified CNDO method and other experimental works on the optical electronic spectrum, in solution and with polarized light in crystals4, ESR experiments3, XPS experiments5 and by Mossbauer6 spectroscopy claim these assignments to be correct.  However there exist evidences towards a different possibility. Chemical reactions of this complex with nucleophiles lead most often to substitution on nitrosyl despite the Fe-NO distance is the shortest one in the complex. Photochemical studies do not support metal orbitals as the HOMO since irradiation on these bands show nitrosyl exchange rather than metal oxidation products. Partially based on these informations E. Wasielewska7 suggested that Manoharan2 assignment was inconsistent with some experimental information. The major inconsistency, following Wasielewska, relates to the observed intensity in the low energy excitations of SNP, which showed rather small intensity transitions, something in disagreement with the usual intense and broad CT bands in similar compounds. A re-interpretation of thermal analysis of SNP, here presented, seems not to support metal orbitals as the HOMO.Bottomley and Grein8 at New Brunswick and Golebiewski and Wasielewska7,9 at Cracow, independently carried out semi-empirical calculations for these transitions. The results are somehow conflicting but showed both, rather different results from the previous SCCC-EHT calculation. Carrying INDO calculations the first authors found a minor contribution of metal d orbitals among the frontier orbitals. The results support the idea that NO ligand strengths the trans-Fe-CN bond and the resulting orbital energy diagram shows the metallic d orbitals too stable. Most of the occupied frontier orbitals are composed by trans and cis CN contributions. These authors claim the CN ligands role essentially as s-donors while NO acts as a strong p-acceptor. As a consequence the first transitions are of a ligand-to-ligand type. According to SINDO calculations carried out by Golebiewski and Wasielewska7,9 however, the HOMO orbitals are of mixed character with about 40% of metal and 60% of cis-CN. All methods agree placing the 13e orbital (p* NO) as the first LUMO, the 3b1 as a 3d (x2-y2), and the 5a1 as the 3d (z2). We report the SINDO assignments inTable 1 compared to the SCCC results. Results obtained with SCCC, INDO and SINDO methods seem to provide rather different representations for the orbital diagram. Therefore the primary question in this study concerns the characterization of the nature of the involved orbitals. Meanwhile this theoretical discussion was taking place, Mossbauer11 experiments indicated a metastable state generated through laser pumping in the 400-530 nm range at 160 K. Below this temperature the metastable state is a long-living state which decays upon irradiation with red light. Violet light does not lead to depopulation of the metastable state. With longer wavelengths or at higher temperatures the metastable states cannot be populated. For a matter of convenience this state will be denoted as HMS-NP (high-temperature metastable state nitroprusside). Following Manoharan and Gray2 this state have been assigned to the 2b2(xy)13e p* (NO) orbital excitation. Due to the remarkable long life of this metastable state its Raman12,13 and IR spectrum15,16, polarized or not, optical UV-visible absorption spectra11,14, Mossbauer6 and ESR spectra have been obtained for many SNP salts. These experiments revealed the existence of a second metastable state below 90 K, hereafter called as LMS-NP (low-temperature metastable state nitroprusside). ESR experiments do not support high-spin paramagnetism but after X-ray irradiation a doublet signal was attributed to the presence of Fe(CN)5NO-3. Based on these results the authors proposed the switching of the first LUMOs, 13e and 22a1(dZ2), one of the remaining few points in agreement over the several articles related to this study. Indeed this is consistent with the reduced Fe-NO bending angle found in the metastable state. Recently similar complexes have been found to exhibit long living electronic states. In the specific

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