Electronic Donation or Steric Contraction: A Spectroscopic and

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ELECTRONIC DONATION OR STERIC CONTRACTION: A SPECTROSCOPIC AND STRUCTURAL ANALYSIS OF MEDIUM-SIZED CONSTRAINED RINGS FOR POTENTIAL LONG- RANGE HYPERCONJUGATION Robert Lee, Bryan Bashrum, Ethan C. Cagle, Jillian Walters, Jake Massey, Monica Zanghi, Carolyn Birchfield, David French, Jessica Joy, Gabriel dos Passos Gomes, and Paul Andrew Wiget J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00979 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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The Journal of Organic Chemistry

ELECTRONIC DONATION OR STERIC CONTRACTION: A SPECTROSCOPIC AND STRUCTURAL ANALYSIS OF MEDIUM-SIZED CONSTRAINED RINGS FOR POTENTIAL LONG-RANGE HYPERCONJUGATION Robert Lee§, Bryan Bashrum§, Ethan C. Cagle, Jillian Walters§, Jake Massey≠, Monica Zanghi≠, Carolyn Birchfield≠, David French, Jessica Joy≠, Gabriel dos Passos Gomes¥,a,Paul A. Wiget§* §Department

of Chemistry and Biochemistry, Samford University, 800 Lakeshore Blvd., Birmingham, Al 35229 of Biological and Environmental Sciences, Samford University, Birmingham, Al ¥Department of Chemistry & Biochemistry, Florida State University, Tallahassee, Fl aPresent address: Department of Chemistry, University of Toronto, Toronto, ON, Canada, Department of Chemistry, The University of Alabama at Birmingham, Birmingham, Al ≠Department

*[email protected]

Keywords: Perlin effect, stereoelectronic effect, nmr, C-H coupling constant, conformational analysis

Abstract. Herein we report the 1JCH analyses, Natural Bond Orbital analyses, and X-Ray crystal structures of a number of C, O, and N constrained tricyclic cycles. These experiments provide access into the nature of the apparent Perlin Effect previously reported in constrained tricyclic cycles, as well as evidence suggesting both steric contraction and long-range hyperconjugation account for the observed 1JCH perturbations. We report a true Perlin Effect of 10.9 Hz in an azocane and large steric effect resulting in 1JC-H = 10.9 Hz in a cyclooctane. Introduction. Remote hyperconjugation is a well-established means of controlling stereo- and regioselectivity in organic reactions, as well as having profound effects on reaction kinetics. These effects are typically quantified experimentally by calculating G in dynamic systems from Keq, or evaluated computationally via Natural Bond Orbital (NBO) analyses.1 We recently reported the appearance of long-range hyperconjugation five bonds away from the donor atom as evaluated by NBO

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analysis in a semi-rigid oxocane system (1, Figure 1a) and postulated this donation was the source of apparently anomalous experimental 1JC-H scalar couplings observed therein.2 The perturbation of the difference between geminal 1JC-H scalar couplings (1JC-H) due to remote electronic donations is known as a Perlin Effect (PE).3 Examples of PEs and the hyperconjugative donations

Figure 1. Examples of Perlin Effects.1-3 General numbering scheme used throughout is provided in a for 1. associated with them are given in Figure 1. Note, 1 appears to exhibit a combination of factors such as an anomeric-like donation (Figure 1, b), and the chemical space between an cyclohexanone and a cyclohexane (Figure 1, c & d). Thus, our prior report was of the discovery of the largest Perlin Effect at a non-anomeric position known to-date. This claim was reevaluated by Salome and Tormena in this journal.4a Their computational work suggested the perturbation of scalar couplings was due to steric contraction of the axial C-H bond as opposed to electronic donation into the * of the remote equatorial C-H as proposed in our work. Though this is not an uncommon perspective on such phenomena,4b we were surprised


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as we calculated and reported a 0.73 kcal/mol stabilization energy in that compound associated with n(O)*(C5-Heq) NBO, with concomitant C5-Heq bond elongation and increase in p character, supporting our claim at the time (see Figure 2, boxed). s*C—H

O

s*C—H

O n

O

O

s*C—H

R1

S R1

1

r (Drdel)[Å]

%p

C-Heq C-Hax Estab (DEdel) [kcal/mol]

1.0940 (-0.00046) 1.0878

78.2 77.3

sulfide 1-S size and polarization

amines 4-6 electronegativty and sterics

0.73 (0.630)

C H

s*C—H

O

s*C—H O

C

H

s*C—H

s*C—H

N

s*C—H

ethers 1’, 2 & 3 orbital alignment

R2 R2

R

s*C—H

C

s*C—H O

s*C—H s*C—H

O cyclooctane 7 control group

alkene 8 bound electron donors Figure 2. Proposed compounds for this study, and the stereoelectronic effects they target.

We hypothesized that synthesizing a series of analogs whereby the donor functionality is modified to affect the electronegativity, polorizability, orientation and/or conformation compared to the donor/accepter system in 1, would provide greater insight into the interplay between the competing theories presented on the origins of these Perlin Effects (see Figure 2). Herein we discuss the synthesis, experimental NMR, conformational analysis and Natural Bond Order (NBO) analysis of a number of synthesized constrained rings in order to probe the extent of donation. We also compare our experimental 1JC-H values with the computational values provided by our aforementioned colleagues. Results and Discussion.


The Journal of Organic Chemistry 1 2 3 4 5 6 7 8 R91 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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We successfully synthesized the tricyclic analogs of 1 seen in TABLE 1 via [3+4] cycloaddition “boat-like” conformation Y

O R2

Y

Conditions

“chair-like” conformation Y n O

O

n

Compound

Y

n

n

1 O 1 1' O 1 2 O 0 3 O 2 4 NBoc 1 5 NCO2Et 1 6 NSO2Ph 1 7 CH 2 1 8 CC(CH 3)2 1 Table 1. [3+4]-cycloadducts synthesized for this study. reactions (see Experimental). All protons and carbons were unambiguously assigned via 1H, 13C, HSQC, HMBC and NOESY NMR for these compounds. The desmethyl analog of 1, compound 1’, was synthesized to compare our data with the 1JC-H values obtained by Salome and to see if the steric bulk of the two methyl groups on furan incorporated into the two prior publications affected the C5-H 1JC-H values. Compounds 2 and 3 were synthesized to compare how orbital misalignment affected the data. In order to compare N versus O donors, we synthesized compounds 4-6 anticipating a means of deprotection affording the free amine. However, ring-opening appeared to be the major product upon analyses of the acid-, base-, or nucleophile-mediated deprotection attempts of 4 and 5. This process is under further investigation and will be reported in time. Recognizing sulfonamides do not share the same resonance delocalization as amides, compound 6 provided access to a system in which the lone pair on nitrogen was properly positioned for through-space donation, while


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maintaining the symmetry of the system not present in 4 and 5. However, concerns over the loss of donation due to inductive or steric effects of the sulfonyl moiety, led us to reduce 5 with LiAlH4 (See Scheme 1). By controlling the equivalents, we were able to produce carboxyethyl aminoalcohol 9 and free aminoalcohol 10 in 27% and 12% yield respectively in separate reactions.

Scheme 1. Reduction of carboxyethyl azocane Compounds 7 lacks a p-orbital donor and is thus a negative electronic control and a positive steric control. Alkene 8 allows for a comparison of donations in protected azocanes 4 and 5 due to their strong amide resonance contributions. Synthesis of alkene 8 thus provides possible electronic or steric controls to explore the possibility of  donations into C5-Heq * or steric effects upon C5Hax. Our efforts to synthesize thiocane donors such as 1-S were unsuccessful. Attempts to use thiophene (Figure 2, R1=R2=H) and 2,5-dimethyl thiophene (R1=Me, R2 = H) as the 4- component in the cycloaddition reaction produced what appeared to be the electrophilic aromatic substitution product, a competing process in such reactions.5 We sought to circumvent this process by using 2,5-diphenylisobenzothiophene6 (R1=Ph, R2 = C4H4) and 2,3,4,5-tetramethylthiophene7 (R1=R2=Me) as 4- components, for they lack the  and  protons necessary for rearomatization. The isobenzothiophene was unreactive and the tetramethylthiophene compound consistently decomposed. Thus, only computational data will be discussed for 1-S (R1 = R2 = H) as our synthetic efforts continue. All compounds synthesized are prone to rapid ring-opening via a Grob-like fragmentation as we previously reported in the parent oxocane between the ketone  and  carbons.8



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In our efforts, it became clear the specific pulse sequence utilized played a larger role in obtaining accurate 1JC-H values than the field strength. In our prior publication, we obtained our data using a standard HSQC pulse sequence with the decoupler turned off, on a 700 MHz instrument; a more powerful magnet than used by our colleagues. This resulted in somewhat poor spectral resolution (likely due to accidental isochrony) and the magnitude of the pseudoaxial 1JCH

had to be extrapolated from the spectrum as discussed in the supporting information for that

publication. In order to obtain more accurate experimental 1JC-H values, we utilized the PerfectCLIP HSQC pulse sequence developed by Parella and performed the experiments on either a 500MHz, 600MHz, 700MHz or 850MHz instrument. This pulse sequence provides “pure in-phase cross peaks with respect to 1JCH and JHH, irrespective of the experiment delay optimization. In addition, peak volumes are not attenuated by the influence of JHH, rendering practical issues such as phase correction, multiplet analysis, and signal integration more appropriate.”9 Upon analysis of 1’ we obtained 1JC-H value of 9.0Hz; quite close to 7.8Hz in the methyl analog 1. These values represent the largest 1JC-H values for a methylene remote to a donor atom to our knowledge at the time. Table 2 provides a comparison of our experimental 1JC-H values and those

calculated

for

analogous

compounds


by

Salome.

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Compound

axial 1

J C-H (Hz)

Experimental equitorial 1

J C-H (Hz)

D J C-H 1

axial 1

J C-H (Hz)

Calculated4 equitorial 1

J C-H (Hz)

D J C-H 1

14.1 7.8 138.8 124.7 9.0 138.8 124.7 14.1 7.5 a 8.6 3 130.2 121.6 4 125.5 124.9 0.6 134.2 126.6 7.6 5 132.6 129.2 3.4 134.2 126.6 7.6 6 133.3 124.3 9.0 134.2 126.6 7.6 7 131.0 129.0 2.0 128.9 128.4 0.5 8 134.5 123.6 10.9 9 122.2 116.4 5.8 10 133.0 122.1 10.9 133.7 126.4 7.3 Table 2. Comparison of experimental C5-H coupling constants and apparent Perlin Effects (1JCH) and those calculated for analogous constrained 8-membered rings by Salome.4 aThe protons of C4 on 2 and C6 on 3 are not aligned with the donor oxygen – axial here is defined as cis to oxygen and equatorial is trans to oxygen. 1 1' 2a

133.0 133.8 140.4

125.3 124.8 132.9

Our data suggests the axial proton in proximity to the donor (C5-Hax) is compressed due to the larger 1JC-H values in all substrates supporting much of Salome’s claim. The PEs of 4 and 5 more closely resemble methylene 7 suggesting the C5 protons are sufficiently removed from both steric compression or long-range donation. Alkene 8 shows a large  1JC-H of 10.9 Hz suggesting a large degree of steric constriction in the C5 axial proton to 1.869 Å with concomitant C5-Heq bond elongation of 1.0964 Å (vida infra). These bond lengths suggest a slightly greater influence of steric strain than plausible donation comparative to 1’. Observing sulfonamide 6 the equatorial CH bond is longer (1JC-Heq = 124.3 Hz) than calculated in either the N-phenyl analog (1JC-Heq = 126.6 Hz) or the unsubstituted azocane (1JC-Heq = 126.4 Hz), presumably due to remote hyperconjugation (vida infra). Additionally, when comparing synthesized aminoalcohol 10 with the azocane calculations, both the degree of C5-Hax compression and C5-Heq elongation are not well modeled when compared with the experimental values of 1JC-H. Concerns over comparing the aminoketone



6 to aminoalcohol 10 led us to obtain 1JC-H values for cyclohexanol and cyclohexanone (See

compound C6H11OH C6H10O

C5-Hax

C5-Heq

129.6

129.8 129.8

Table 3. Comparison of cyclohexanol (C6H11OH) and cyclohexanone (C6H10O) in CD2Cl2 @ 298K. Table 3). The similarity between cyclohexanol and cyclohexanone at C5 suggests this modification has little impact on the PEs at this position, affording additional confidence in this interpretation of the data for 6 and 10. Compound 7 shows little PE at all, and exhibits 1JC-H values dissimilar to cyclohexanone or any other 8-membered cycle we synthesized, save carboxyethyl azocane 5. The similarity between 5 and 7 implies a similar electronic environment surrounding the methylenes in both compounds. Alkene 8 exhibits the largest 1JC-Hax (134.5 Hz) of all the 8-membered cycles suggesting a large degree of compression. As we are seeking to ascertain the cause of the PEs we plotted 1JC-Heq versus 1JC-Hax (Figure SI-90) and found that 1, 1’, 6, 8, and 10 were highly similar in coupling constants. Of the calculated coupling constants, only the NH and NPh analogs were similar to experimental values for 1, 1’ and 6, and calculations for 7 were similar to those determined experimentally. Compound 1 varied significantly from the experimental 1JC-Hax value.


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The syntheses of oxopane 2 and oxonane 3 were carried out to evaluate field effects the donor oxygen would have when donor and acceptor orbitals are misaligned compared to oxocane

Figure 3: Side-view of 1’, 2, and 3 with "equatorial" proton emphasized for reference. (top). Top-down view of the cyclic ether portion of 1, 2, and 3 with * orbitals emphasized for reference. 1’ (see Figure 3). The protons of interest in oxopane 2 both have larger coupling constants than either 1’, or 3, consistent with their proximity to oxygen. Their Perlin Effect,  1JC-H = 7.5 Hz, is still quite large for a non-anomeric methylene. In oxonane 3, the methylenes are considerably further away from the donor oxygen yet experience a comparable apparent PE of 8.6 Hz. This is intriguing as this compound likely isn’t in the conformation for donation. These results suggest either orbital alignment is not as necessary for donation hypothesized, or the proximity to the donor is all that is necessary to convolute 1JC-H interpretation. Varying the solvent had minimal effect on the coupling constants for 1’ when experiments were

performed

at

the

same

temperature


(See

Table

4).


1'

C6H11OH

C6H10O

signal

CD 2Cl2 298 K 283 K

C5-H ax

134.2

C5-H eq

Page 10 of 28

298 K

toluene-d 8 283 K

277 K

134.1

133.7

-

-

124.0

127.7

122.8

-

-

D1J CH

10.2

6.4

10.9

-

-

C5-H ax

129.6

132.8

126.9

122.7

127.6

C5-H eq

129.8

129.8

123.3

123.0

129.8

D1J CH

0.2

3.0

3.6

0.3

2.2

C5-H

129.8

123.6

127.8

121.2

-

Table 4. Evaluation of solvent and temperature effects between 1', cyclohexanol and cyclohexanone. The CH coupling constants were however fairly different at different temperatures presumably due to the changes in Boltzmann distribution of conformers. No discernable trend was observed between 1’, cyclohexanone and cyclohexanol. These interesting data warrant further investigation which will be presented in future works. Unfortunately we have not been able to obtain nmr data below the temperature necessary to slow conformational changes at this time. We solved crystal structures for compounds 1, 1’, 2, 6 and 7 (Figure 3).

Figure 4. Top: ChemDraw structures of compounds 1’, 1, 2, 6, and 7 depicting conformations and donor-acceptor intramolecular bond distances. Bottom: X-Ray crystal structures and pertinent bond distances and dihedral angles. Atomic displacement ellipsoids generated at 50% probability.


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These structures show how the increase in size of the donor atom or group affect the overall conformation of the ring system in the solid phase. In the two oxocanes 1 and 1’ the oxygen atom is slightly closer to C5 than it is in the nitrogen analog 6. The C4-C5 dihedral angle is also compressed by ~4o in the azocane system. These features support the claim steric or Lewis interactions are strongly contributing to the difference in observed 1JC-H . However, the Perlin Effects in both 1’ and 6 are identical. This could suggest the increased distance between the donor and acceptor are perhaps increasing the ability of through space hyperconjugation via better orbital alignment. Compound 10, which incorporates a methyl substituent in place of the sulfonylphenyl moiety, shows near identical 1JC5-Hax values to 6 but a smaller 1JC5-Heq value, resulting in an apparent Perlin Effect of 10.9 Hz. This represents the largest Perlin Effect to date in a non-anomeric methylene. If one considers the larger van der Waal radius of the sulfonylphenyl group (A = 2.94 kcal/mol) compared to that of a methyl group (A = 1.74 kcal/mol)10 this result is surprising, as the compression of C5-Hax should be greater (as evidenced by a larger 1JC5-Hax) with the larger substituent in close proximity, thus increasing the PE compared to 10. Oxopane 2 shows a donor oxygen ~0.3Å further away due to the inherent conformational constraints in the smaller ring. In the methylene “donor” compound 7, the steric bulk has increased to such a degree that the Lewis interactions have forced a conformational change in the structure to the chair-like cyclooctane.



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Y

Y H H

O

O

H

H chair-like donation impossible

boat-like donation possible

Figure 5. Conformational requirements for donation As seen in Figure 5,

the molecules

must adopt the proper conformation in solution in order to achieve long-range orbital overlap to be possible. In the chair-like conformation, the donor and acceptor are too far from one another to achieve orbital overlap. Thus, a conformational analysis of the synthesized compounds is needed to determine to what extent the compounds are in the boat-like conformation necessary for donation. Table 5 provides G, Keq and the percent of each compound in the boat-like conformation for all compounds.

Compound

G

Keq

1'

2.5

68.3

% boatlike 99

1

1.7

17.7

95

2 3 4 5 6

single conformer -0.7 1.1 1.1 -0.2

0.3 6.4 6.4 0.7

23 87 87 42


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7 8 9 10

-0.5 0.6 -0.2 0

0.4 2.8 0.7 1.0

30 73 42 50

S-1 2.4 57.7 98 Table 5. Calculated G values (boat-like  chair-like, kcal/mol) for each compound and their associated Boltzmann Distributions @ 298K. Conformer estimates for compounds 3 and 7 suggest the wrong conformer (chair-like conformation) for remote donation dominates the equilibrium providing controls. Compound 8 is thought to hold the proper conformation and utilize -electrons for donation instead of a lone pair. Compounds 1, 1’, 4, 5, 8, and S-1 heavily favor the boat-like conformation. Compounds 6, 9, and 10 have essentially equal proportions of each conformer. All compounds show only one set of signals in their 1H NMR, thus the observed signals (and 1JCH

values) represent weighted averages of the conformations present (suggesting a low energy

barrier for interconversion). The conformation of 10 was unambiguously determined via NOESY NMR (see Figure SI-77), with the only in-phase signal being that of the OH. Compounds 4 and 5 theoretically provide examples of -donation electronically similar to alkene 8 due to resonance delocalization. Unfortunately, the strong amide resonance contribution renders the constrained rings asymmetrical, complicating J-analysis. Compound 9 showed an out-of-phase signal in the NOESY, however this signal could be either C5-Hax or O-H as these signals are isochronous. Comparing the NMR predictions for the two conformations of 4, 5, 8 and 10 and the experimental NMR of these compounds, suggests they exist predominantly in the boat-like 8-membered ring conformation necessary for remote donation. 4 also appears to have adopted a stable half-chairlike conformation, with a C4-Heq-C5-Hax dihedral angle of -20o. Compound 5 showed an out-ofphase signal in the NOE between C5-Hax and one of the C2 protons. This suggests the cyclohexanone is involved in a rapid equilibrium between the chair-like oxocane and a twist-boat-



like conformer. These differences are presumably due to reduced steric strain in the carboxyethyl substituent compared with the Boc group. J-analysis of compound 6 suggests it is in a half-chair conformation with C4-H coupling constants of 7.3Hz and 2.0Hz representing a near-eclipsed synperiplanar and anticlinal relationship to the C5 protons. The crystal structure also strongly supports this conformation, though the lattice energy may overcome the solution-phase preference for the chair-like conformer. We then turned to NBO analysis to estimate the stabilization (and deletion) energies, C5-H bond lengths, and p-orbital contribution in the plausible conformations of these compounds.


Page 14 of 28

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1

r (Drdel)[Å]

%p

1'

r (Drdel)[Å]

%p

2

r (Drdel)[Å]

%p


1.0940 (-0.00046) 1.0878

78.2 77.3


1.0937 (-0.00091) 1.0873

78.2 77.3


1.0947 (-0.00091) 1.0864

77.0 76.4

3

r (Drdel)[Å]

%p

4

r (Drdel)[Å]

%p

5

r (Drdel)[Å]

%p


1.094 (-0.00007) 1.0913

78.8 78.2


1.0935 (-0.00115) 1.0883

78.4 77.3


1.0936 (-0.00133) 1.0882

78.3 77.4

0.73 (0.630)

0.59 (0.4915)

0.77 (0.642)

0.77 (0.908)

0.77 (0.7105)

0.93 (0.902)

Electronic Donation or Steric Contraction: A Spectroscopic and

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