Observation of (31)P-(31)P Indirect Spin-Spin Coupling in Copper-Bis(phosphine) Complexes by Two-Dimensional Solid-State NMR

The first observations of (31)P-(31)P indirect spin-spin (J) coupling in copper(I) phosphine complexes are reported for solid Cu(PPh(3))(2)X (X = NO(3)(-), BH(4)(-)). Values of (2)J((31)P,(31)P), 157 +/- 5 and 140 +/- 5 Hz for Cu(PPh(3))(2)NO(3) and Cu(PPh(3))(2)BH(4), respectively, have been obtained from two-dimensional (2D) J-resolved (31)P NMR spectra obtained under slow magic-angle spinning (MAS) conditions. In both complexes, the two phosphine ligands are crystallographically equivalent; thus, the two (31)P nuclei have identical isotropic chemical shifts. Under rapid sample spinning conditions, the (31)P MAS NMR spectra exhibit relatively sharp overlapping asymmetric quartets arising from (1)J((63/65)Cu,(31)P) and residual (63/65)Cu-(31)P dipolar interactions. No evidence of (2)J((31)P,(31)P) is apparent from the spectra obtained with rapid MAS; however, under slow MAS conditions there is evidence of homonuclear J-recoupling. Peak broadening due to heteronuclear dipolar interactions precludes measurement of (2)J((31)P,(31)P) from standard 1D (31)P MAS NMR spectra. It is shown that this source of broadening can be effectively eliminated by employing the 2D J-resolved experiment. For the two copper(I) phosphine complexes investigated in this study, the peak widths in the f(1) dimension of the 2D J-resolved (31)P MAS NMR spectra are about three times narrower than those found in the corresponding 1D (31)P MAS NMR spectra.     

(3h)J((15)N-(31)P) spin-spin coupling constants across N[bond]H....O[bond]P hydrogen bonds

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to evaluate three-bond (15)N-(31)P coupling constants ((3h)J(N[bond]P)) across N[bond]H....O[bond]P hydrogen bonds in model cationic and anionic complexes including NH(4)(+):OPH, NH(4)(+):OPH(3), NH(3):(-)O(2)PH(2), NFH(2):(-)O(2)PH(2), and NF(2)H:(-)O(2)PH(2). Three-bond coupling constants can be appreciable when the phosphorus is P(V), but are negligible with P(III). (3h)J(N[bond]P) values in complexes with cyclic or open structures are less than 1 Hz, a consequence of the nonlinear arrangement of N, H, O, and P atoms. For complexes with these structures, (3h)J(N[bond]P) may not be experimentally measurable. In contrast, complexes in which the N, H, O, and P atoms are collinear or nearly collinear have larger values of (3h)J(N[bond]P), even though the N[bond]P distances are longer than N[bond]P distances in cyclic and open structures. In linear complexes, (3h)J(N[bond]P) is dominated by the Fermi-contact term, which is distance dependent. Therefore, N[bond]P (and hydrogen-bonding N[bond]O) distances in these complexes can be determined from experimentally measured (15)N-(31)P coupling constants.     

Mechanochemical and solution synthesis, and crystal structures and IR and solid-state (CPMAS) NMR spectroscopy of some bis(triphenylphosphine)silver(I) mono- and di-hydrogencitrate systems

The complex [(Ph(3)P)(2)Ag(H(2)cit)]·EtOH (1; H(2)cit(-) = dihydrogencitrate = C(6)H(7)O(7)(-)) contains [(Ph(3)P)(2)Ag(H(2)cit)] molecules in which the silver atom is coordinated to two PPh(3) molecules and the two oxygen atoms of one of the 'terminal'/1-carboxylate groups of the dihydrogencitrate group. The molecules form centrosymmetric hydrogen-bonded dimers in the solid. In [{(Ph(3)P)(2)Ag}(2)(Hcit)], (2), unsymmetrical deprotonation of the citrate grouping is found, from the 1- and 3- (i.e. terminal and central) carboxylates: [(Ph(3)P)(2)Ag(O(2)CCH(2)C(OH) (CH(2)COOH)CO(2))Ag(PPh(3))(2)]. The above complexes, as well as [(Ph(3)P)(3)Ag(H(2)cit)] (3) were prepared via conventional solution methods, involving the reaction of trisilver(I) citrate, citric acid and triphenylphosphine, and by a mechanochemical method involving the reaction of silver(I) oxide, citric acid and triphenylphosphine. IR studies of 1-3 show the presence of coordinated carboxylate and free carboxylic acid groups in the mono- and di-hydrogencitrate ligands, and the formation of 2 from 1 shows that dihydrogencitrate deprotonation can occur upon dissolution of 1 in protic solvents. High-field (9.40 T) (31)P CPMAS NMR spectra were recorded and analysed, yielding heteronuclear (1)J((107/109)Ag,(31)P) and homonuclear (2)J((31)P,(31)P) spin-spin coupling constants.     

A DFT study of the interresidue dependencies of scalar J-coupling and magnetic shielding in the hydrogen-bonding regions of a DNA triplex.

Scalar coupling constants and magnetic shieldings in the imino hydrogen-bonding region of Hoogsteen-Watson-Crick T.A-T and C(+).G-C triplets have been calculated as a function of the distance between proton donor and acceptor nitrogen atoms. The Fermi contact contributions to (h2)J((15)N-H...(15)N), (1)J((15)N-(1)H), and (h1)J((1)H...(15)N) were computed using density functional theory/finite perturbation theory (DFT/FPT) methods for the full base triplets at the unrestricted B3PW91/6-311G level. Chemical shifts delta((1)H) and delta((15)N) were obtained at the same level using the gauge including atomic orbital (GIAO) method for magnetic shielding. All three scalar couplings and all three chemical shifts are strongly interrelated and exhibit monotonic changes with base pair separation. These correlations are in conformity with experimental data for a 32-nucleotide DNA triplex. The results suggest that both chemical shifts and coupling constants can be used to gain information on H-bond donor-acceptor distances in nucleic acids. In addition to the DFT/FPT calculations, a simple three-orbital model of the N-H...H bond and a sum-over-states analysis is presented. This model reproduces the basic features of the H-bond coupling effect. In accordance with this model and the DFT calculations, a positive sign for the (h2)J(NN) coupling is determined from an E.COSY experiment.     

Vibrational state-dependent predissociation dynamics of ClO (A (2)Pi(3/2)): Insight from correlated fine structure branching ratios

We have studied the v'-dependent predissociation dynamics of the ClO A (2)Pi(3/2) state using velocity-map ion-imaging. Experimental final correlated state branching ratios, i.e. Cl((2)P(J=3/2,1/2)) + O((3)P(J=2,1,0)) channels, have been measured for v' = 6-11. We find that the branching ratios are highly variable and depend strongly on v', providing a window into the v'-dependent predissociation mechanism. A comparison of the experimental results with the recent model of Lane et al. (I. C. Lane, W. H. Howie and A. J. Orr-Ewing, Phys. Chem. Chem. Phys., 1999, 1, 3087) in both the diabatic and adiabatic limits suggests that the dynamics are closer to the diabatic limit. The overall Cl((2)P(J)) branching ratios are in good agreement with the diabatic model results. There are significant differences, however, between theory and experiment at the correlated state level, demonstrating the sensitivity of correlated measurements to the role of the exit channel coupling in the predissociation dynamics. The results highlight the need for more sophisticated quantum dynamical calculations to describe the correlated fine structure branching ratios in this system.     

Comparing main group and transition-metal square-planar complexes of the diselenoimidodiphosphinate anion: a solid-state NMR investigation of M[N(iPr2PSe)2]2 (M = Se, Te; Pd, Pt)

A comparison of the square-planar complexes of group 10 (Pd(II), Pt(II)) and 16 (Se(II), Te(II)) centers with the tetraisopropyldiselenoimidodiphosphinate anion, [N((i)Pr2PSe)2](-), is made on the basis of the results of a solid-state (31)P, (77)Se, (125)Te, and (195)Pt NMR investigation. Density functional theory calculations of the respective chemical shift and (14)N electric field gradient tensors in these compounds complement the experimental results. The NMR spectra were analyzed to determine the respective phosphorus, selenium, tellurium, and platinum chemical shift tensors along with numerous indirect spin-spin coupling constants. Special attention was given to observed differences in the NMR parameters for the transition metal and main-group square-planar complexes. Residual dipolar coupling between (14)N and (31)P, not observed in the solid-state (31)P NMR spectra of the Pd(II) and Pt(II) complexes, was observed at 4.7 and 7.0 T for M[N((i)Pr 2PSe)2]2(M = Se, Te) yielding average values of R((31)P, (14)N)eff = 890 Hz, CQ((14)N) = 2.5 MHz, (1) J( (31)P, (14)N) iso= 15 Hz, alpha = 90 degrees , beta = 17 degrees . The span, Omega, and calculated orientation of the selenium chemical shift tensor for the diselenoimidodiphosphinate anion is found to depend on whether the selenium is located within a pseudoboat or distorted-chair MSe 2P 2N six-membered ring. The largest reported values of (1)J((77)Se, (77)Se) iso, 405 and 435 Hz, and (1)J((125)Te, (77)Se)iso, 1120 and 1270 Hz, were obtained for the selenium and tellurium complexes, respectively; however, in contrast a correspondingly large value of (1)J((195)Pt, (77)Se)iso was not found. The chemical shift tensors for the central atoms, Se(II) and Te(II), possess positive skews, while for Pt(II) its chemical shift tensor has a negative kappa. This observed difference for the shielding of the central atoms has been explained using a qualitative molecular orbital approach.     

Accordion BIRD-HMBC experiments: improved one-bond correlation suppression in accordion heteronuclear multiple-bond correlation-type experiments

Modifications of the BIRD-HMBC experiment, in which the accordion principle and a constant time period have been introduced, are presented. With the simple accordion BIRD-HMBC, the use of a BIRD(r, X) element in the middle of the variable long-range couplings evolution period efficiently eliminates unwanted (1)J(CH) signals and samples a broad range of long-range heteronuclear couplings. While the accordion BIRD-HMBC experiment shows reduced F(1) modulation, superior suppression of unwanted (1)J(CH) signals and sensitivity comparable to the ACCORD-HMBC, the addition of another constant time period suppresses F(1) modulation due to (1)H-(1)H coupling, while preserving the excellent suppression of unwanted (1)J(CH) signals. The constant-time (CT) accordion BIRD-HMBC experiment combines sensitivity similar to the CIGAR-HMBC and superior suppression of unwanted (1)J(CH) signals. Results of identically optimized ACCORD-HMBC, accordion BIRD-HMBC, CT accordion BIRD-HMBC and CIGAR-HMBC experiments performed on a sample of strychnine are compared and discussed.     

A solid-state NMR investigation of single-source precursors for group 12 metal selenides; M[N(iPr2PSe)2]2 (M = Zn, Cd, Hg)

The first solid-state NMR investigation of dichalcogenoimidodiphosphinato complexes, M[N(R(2)PE)(2)](n), is presented. The single-source precursors for metal-selenide materials, M[N((i)Pr(2)PSe)(2)](2) (M = Zn, Cd, Hg), were studied by solid-state (31)P, (77)Se, (113)Cd, and (199)Hg NMR at 4.7, 7.0, and 11.7 T, representing the only (77)Se NMR measurements, and in the case of Cd[N((i)Pr(2)PSe)(2)](2)(113)Cd NMR measurements, to have been performed on these complexes. Residual dipolar coupling between (14)N and (31)P was observed in solid-state (31)P NMR spectra at 4.7 and 7.0 T yielding average values of R((31)P,(14)N)(eff) = 880 Hz, C(Q)((14)N) = 3.0 MHz, (1)J((31)P,(14)N)(iso) = 15 Hz, alpha = 90 degrees , beta = 26 degrees . The solid-state NMR spectra obtained were used to determine the respective phosphorus, selenium, cadmium, and mercury chemical shift tensors along with the indirect spin-spin coupling constants: (1)J((77)Se,(31)P)(iso), (1)J((111/113)Cd,(77)Se)(iso), (1)J((199)Hg,(77)Se)(iso), and (2)J((199)Hg,(31)P)(iso). Density functional theory magnetic shielding tensor calculations were performed yielding the orientations of the corresponding chemical shift tensors. For this series of complexes the phosphorus magnetic shielding tensors are essentially identical, the selenium magnetic shielding tensors are also very similar with respect to each other, and the magnetic shielding tensors of the central metals, cadmium and mercury, display near axial symmetry demonstrating an expected deviation from local S(4) symmetry.     

Three-bond sugar-base couplings in purine versus pyrimidine nucleosides: a DFT study of Karplus relationships for (3)J(C2/4-H1') and (3)J(C6/8-H1') in DNA

(3)J(C2/4-H1') and (3)J(C6/8-H1') scalar spin-spin coupling constants have been calculated for deoxyadenosine, deoxyguanosine, deoxycytidine, and deoxythymidine as functions of the glycosidic torsion angle chi by means of density functional theory. Except for deoxythymidine, (3)J(C2/4-H1') depends little on the base type. On the contrary, (3)J(C6/8-H1') follows the usual trans to cis ratio ((3)J(C-H(cis)) < (3)J(C-H(trans))) for purine nucleosides, but reveals the opposite relation ((3)J(C-H(cis)) > (3)J(C-H(trans))) for pyrimidine nucleosides. Our results compare well with the experiment for deoxyguanosine and predict a novel trend in the case of pyrimidine bases for which no NMR results are available in the syn region. A breakdown of the key Fermi contact part of (3)J(C6/8-H1') into MO contributions rationalizes this trend in terms of an unusual coupling mechanism in the syn orientation that is very effective for pyrimidine nucleosides and considerably weaker for purine nucleosides.     

Eremophilane esters of Robinsonecio gerberifolius and their rearranged products. Study of the coupling constants (2)J(H, H), (3)J(H, H) and (4)J(H, H)

In the course of the basic hydrolysis of four eremophilane esters isolated from Robinsonecio gerberifolius, some rearrangements, eliminations, and additions occurred. Five compounds were obtained, three of them not previously described. Additionally, a new sesquiterpene was produced by autooxidation of compound 1. The (1)H and (13)C NMR spectra of these compounds were completely assigned by utilization of HMQC, HMBC, COSY, DEPT, and NOESY techniques. The long-range coupling constants of the peroxide 10 are reported, and all its coupling constants (2)J(H, H), (3)J(H, H), and (4)J(H, H) are calculated at the B3LYP/6-31G(d,p) level of theory. Their magnitude is explained in terms of electronic delocalization and the additivity of stereoelectronic effects.     

Tautomerism in the solid state and in solution of a series of 6-aminofulvene-1-aldimines

To study systems able to sustain intramolecular proton-transfer, we have prepared a series of six aminofulvene aldimines including several labeled with (15)N and (2)H. These compounds show coupling constants through the hydrogen bond, (1h)J((15)N- (1)H) and (2h)J((15)N-(15)N). The position of the tautomeric equilibria, i.e., on what nitrogen atom is the proton, was determined in the solid state and in solution. The crystal structure of N[[5-[(phenylamino)methylene]-1,3-cyclopentadien-1-yl]methylene]pyrrole-1-amine (3) has been determined by X-ray analysis. In solution, both N-H and C-H tautomers were observed and their structures assigned by NMR spectroscopy. Particularly useful is the value of the (1)J((15)N-(1)H) coupling constant.     

Silver(I) complexes of a sterically demanding fluorinated triazapentadienyl ligand [N((C3F7)C(Dipp)N)2]- (Dipp = 2,6-diisopropylphenyl)

Sterically demanding triazapentadiene [N((C3F7)C(Dipp)N)2]H affords the isolation of thermally stable, two- and three-coordinate silver complexes. The free ligand [N((C3F7)C(Dipp)N)2]H has a W-shaped ligand backbone in the solid state.[N((C3F7)C(Dipp)N)2]H reacts with silver(I) oxide in acetonitrile leading to CH(3)CNAg [N((C3F7)C(Dipp)N)2]HIt features a two-coordinate silver center and a kappa(1)-coordinated triazapentadienyl ligand. This silver acetonitrile complex serves as an excellent precursor to obtain thermally stable, silver isocyanide t-BuNCAg [N((C3F7)C(Dipp)N)2]Hand silver phosphine [[N((C3F7)C(Dipp)N)2]HAgPPh(3) adducts. IR spectroscopic data for the silver(I) isocyanide t-BuNCAg [N((C3F7)C(Dipp)N)2]Hshows nu(CN) at 2219 cm(-)(1). The silver ion coordinates to the triazapentadienyl ligand via the central nitrogen atom. The silver PPh(3) adduct,[N((C3F7)C(Dipp)N)2]HAgPPh(3), was synthesized by treating CH3CNAg [N((C3F7)C(Dipp)N)2]Hwith PPh(3). It displays relatively large Ag-P coupling in the (31)P NMR spectrum. The triazapentadienyl ligand in[N((C3F7)C(Dipp)N)2]HAgPPh(3) acts as a chelating kappa(2)-donor. The Ag-P bond is relatively short (2.3487(10) A).     

Ab initio study of the influence of trimer formation on one- and two-bond spin-spin coupling constants across an X-H-Y hydrogen bond: AH:XH:YH3 complexes for A, X = 19F, 35Cl and Y = 15N, 31P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor.     

Trans-hydrogen-bond (h2)J(NN) and (h1)J(NH) couplings in the DNA A-T base pair: natural bond orbital analysis

Natural bond orbital (NBO) analysis described here demonstrates that trans-hydrogen-bond (trans-H-bond) NMR J couplings in the DNA A-T base pair, h2JNN and h1JNH, are determined largely by three terms: two Lewis-type contributions (the single-orbital contribution from the adenine lone pair and the contribution from the sigmaN3H3 natural bond orbital of the thymine ring) and one contribution from pairwise delocalization of spin density (between the lone pair in adenine and the sigma* antibonding orbital linking N3 and H3 of thymine). For h2JNN coupling, all three contributions are positive, whereas for h1JNH coupling, the delocalization term is negative, and the other two terms are positive, resulting in a small net positive coupling constant. This result rationalizes the experimental findings that the two-bond coupling (h2JNN approximately 9 Hz) is larger than the one-bond coupling (h1JNH approximately 3 Hz) and demonstrates that the same hyperconjugative and steric mechanisms that stabilize the H-bond are involved in the transmission of J coupling information. The N1...H3-N3 H-bond of the DNA A-T base pair is found to exhibit significant covalent character, but steric effects contribute almost equally to the trans-H-bond coupling.     

Quantum state-to-state dynamics for the quenching process of Br(2P1/2) + H2(v(i) = 0, 1, j(i) = 0)

Quantum state-to-state dynamics for the quenching process Br((2)P(1/2)) + H(2)(v(i) = 0, 1, j(i) = 0) → Br((2)P(3/2)) + H(2)(v(f), j(f)) has been studied based on two-state model on the recent coupled potential energy surfaces. It was found that the quenching probabilities have some oscillatory structures due to the interference of reflected flux in the Br((2)P(1/2)) + H(2) and Br((2)P(3/2)) + H(2) channels by repulsive potential in the near-resonant electronic-to-vibrational energy transfer process. The final vibrational state resolved integral cross sections were found to be dominated by the quenching process Br((2)P(1/2)) + H(2)(v) → Br((2)P(3/2)) + H(2)(v+1) and the nonadiabatic reaction probabilities for Br((2)P(1/2)) + H(2)(v = 0, 1, j(i) = 0) are quite small, which are consistent with previous theoretical and experimental results. Our calculated total quenching rate constant for Br((2)P(1/2)) + H(2)(v(i) = 0, j(i) = 0) at room temperature is in good agreement with the available experimental data.     

Electronic influences on 3J(C,H) coupling constants via -S-, -S(O)- and -SO2-: their determination, calculation and comparison of detection methods

3J(C,H) coupling constants via a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone, were detected experimentally and calculated by quantum mechanical methods. In the first series (1-3) the coupling between a hydrogen, bonded to an sp3 carbon, and an sp2 carbon is treated; the second series (4-6) deals with the coupling between a hydrogen, bonded to an sp3 carbon, and an sp3 carbon. Different pulse sequences (broadband HMBC, SelJres, 1D HSQMBC, J-HMBC-2, selective J-resolved long-range experiment and IMPEACH-MBC) proved to be useful in determining the long-range 3J(C,H) coupling constants. However, the dynamic behaviour of two of the compounds (4 and 6) led to weighted averages of the two coupling constants expected (concerning equatorial and axial positions of the corresponding hydrogens). DFT calculations proved to be useful to calculate not only the 3J(C,H) coupling constants but also the different contributions of FC, PSO, DSO and SD terms; the calculation of the Fermi contact term (FC) was found to be sufficient for the correct estimation of 3J(C,H) coupling constants.     

HMBC-RELAY: a combined technique for the differentiation of simultaneously detected 2J(C,H) and (n)J(C,H) connectivities

We present a new pulse sequence that detects simultaneously (n)J(C,H) and 2J(C,H) connectivities. The corresponding coherences are created along independent pathways and therefore can be separated into two different subspectra. One spectrum is to show all (n)J(C,H) connectivities and the other is to show exclusively 2J(C,H) connectivities. In contrast to the previously published 2J/(n)J experiment, this sequence detects the 2J(C,H) connectivities via a C,H,H-RELAY pathway leading to an intensification of the 2J(C,H) signals. Strictly, the 2J(C,H) spectrum does not show 2J(C,H) but 3J(H,H) coupling interactions within 13CH(k)-12CH(l) fragments. Therefore, 2J(C,H) signals can appear even if the corresponding 2J(C,H) coupling constant is zero.     

Measurement of 1J(Ni,Calpha(i)), 1J(Ni,C'i-1), 2J(Ni,Calpha(i-1)), 2J(H(N)i,C'i-1) and 2J(H(N)i,Calpha(i)) values in 13C/15N-labeled proteins

Use of partial or selective (13)C/(15)N labeling of specific amino acid residues in a given protein to measure the values of (1)J((15)N(i),(13)C(alpha) (i)), (2)J((1)H(N),(13)C(alpha) (i)), (2)J((15)N(i),(13)C(alpha) (i-1)), (1)J((15)N(i),(13)C'(i-1)) and (2)J((1)H(N),(13)C'(i-1)) is described. This was achieved by recording a sensitivity-enhanced 2D [(15)N-(1)H] HSQC experiment, without mixing the spin states of C(alpha) and C' during the course of entire experiment.     

How to insulate a reactive site from a perfluoroalkyl group: photoelectron spectroscopy, calorimetric, and computational studies of long-range electronic effects in fluorous phosphines P((CH(2))(m)(CF(2))(7)CF(3))(3)

This study advances strategy and design in catalysts and reagents for fluorous and supercritical CO(2) chemistry by defining the structural requirements for insulating a typical active site from a perfluoroalkyl segment. The vertical ionization potentials of the phosphines P((CH(2))(m)R(f8))(3) (m = 2 (2) to 5 (5)) are measured by photoelectron spectroscopy, and the enthalpies of protonation by calorimetry (CF(3)SO(3)H, CF(3)C(6)H(5)). They undergo progressively more facile (energetically) ionization and protonation (P(CH(2)CH(3))(3) > 5 > 4 approximately equal to P(CH(3))(3) > 3 > 2), as expected from inductive effects. Equilibrations of trans-Rh(CO)(Cl)(L)(2) complexes (L = 2, 3) establish analogous Lewis basicities. Density functional theory is used to calculate the structures, energies, ionization potentials, and gas-phase proton affinities (PA) of the model phosphines P((CH(2))(m)()CF(3))(3) (2'-9'). The ionization potentials of 2'-5' are in good agreement with those of 2-5, and together with PA values and analyses of homodesmotic relationships are used to address the title question. Between 8 and 10 methylene groups are needed to effectively insulate a perfluoroalkyl segment from a phosphorus lone pair, depending upon the criterion employed. Computations also show that the first carbon of a perfluoroalkyl segment exhibits a much greater inductive effect than the second, and that ionization potentials of nonfluorinated phosphines P((CH(2))(m)CH(3))(3) reach a limit at approximately nine carbons (m = 8).