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Pseudohalogen chemistry. Part 12. NMR and IR studies of substituent effects in 4-substituted thiocyanatobenzenes
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SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 1703 1713
Pseudohalogen chemistry. Part 12. NMR and IR studies of substituent effects in 4-substituted thiocyanatobenzenes A. Bangher, R.G. Guy *, Y. Pichot, J.M. Sillence, C.J. Steel, F.J. Swinbourne, K. Tamiatti School of Natural Sciences, The University of Hertlbrdshire, Hatfield, Hert/brdshire A L 10 9AB, UK
Received 14 November 1994; accepted 21 December 1994
Abstract
~H and ~3C N M R chemicals shifts for 15 4-X-thiocyanatobenzenes have been measured at high dilution in CDCI 3 solution. Substituent chemical shifts for the SCN group were derived from the data for thiocyanatobenzene. Excellent substituent-shift correlations (r > 0.995) were obtained for the SCN carbon using single and dual substituent parameter (SSP and DSP) approaches, and for the ring carbons using SSP, modified DSP, and triple substituent parameter (TSP) approaches. The SCN carbon experiences a reverse substituent effect through both polar and resonance components of the electronic effect of X. Reverse resonance effects were also observed tbr C-2(6) and C-4, and reverse polar effects for C-2(6) and C-3(5). The SCN group causes a 50% enhancement of the normal para SCS values of X at C-I. SSP and DSP substituent-shift correlations were successful for H-3(5) but only fair or poor for H-2(6). IR SCN stretching frequencies were also measured and correlated successfully with substituent parameters using SSP and DSP methods; in contrast to ~SCN, vSCN shows a normal substituent effect.
1. Introduction
There are m a n y literature examples o f the correlation o f nuclear and side-chain IH and 13C N M R chemical shifts in para-substituted benzenes with substituent effects using singleand multi-parameter methods [1]. The effects o f substituents on the I R absorption frequencies of functional groups attached to the benzene ring have been analysed in a similar fashion [2]. These investigations have been instrumental in our understanding o f the nature o f the polar and resonance effects o f substituents on a probe atom or group, their modes o f transmission t h r o u g h the aromatic framework, and their relative contributions [1-3]. In this paper we report the IH and 13C N M R chemical shifts and the S C N I R stretching frequencies of a representative series o f 4-substituted thiocyanatobenzenes ( l a - l o ) . SCN
a:X=H f:X=NO2
b:X=NM~ c:X=NI-~ g:X--COM~ h : X = C 1
d:X=OMe i:X=Br
e:X=M~ |:X=I
k:X=Et
I:X=i-Pr
m:X=NHAe n:X=SCN
o:X=OH
* Corresponding author. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10584 8539(94)01343-1
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A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
The substituents cover a wide range of electronic effects and include a "basis set" to obtain statistically valid correlations [1,2]. The spectra have been recorded in dilute solution in an inert solvent to minimise solute-solute and solvent-solute interactions [1,2], and the resulting data have been correlated with substituent effects using single, dual and triple substituent parameter (SSP, DSP and TSP) methods [4]. A few earlier reports of the N M R [5] and IR [6] spectra of aromatic thiocyanates were of limited value to this study, since insufficient substituents were studied, concentrations were not recorded, instrument sensitivity was low, or spectral analyses were incomplete.
2. Experimental 2.1. Syntheses
Compounds la, l d - l l , and In were prepared from the corresponding amine by a Sandmeyer reaction using iron(III) thiocyanate as catalyst and were separated from the isothiocyanato co-product by chromatography on a silica gel column [7]. Compounds lb, le and 1o were prepared by reaction of thiocyanogen with the appropriate amine or phenol [7]. Compound lm was prepared by acetylation of le using acetyl chloride. All the compounds have been characterised previously [8]. 2.2. Spectra The 13C N M R spectra were obtained on a Bruker AC 250 spectrometer at a frequency of 62.896 MHz with a pulse width of 2.7 las (45°), using a spectral width of 15.625 kHz and 6 4 K data points to give a maximum attainable resolution of 0.008ppm. The ~H spectra were similarly obtained at a frequency of 250.133 MHz, with a pulse width of 3.0 ~ts (30°), a spectral width of 5.000 KHz and 32 K data points giving a maximum attainable resolution of 0.001 ppm. The spectra were recorded on samples at 3% w/v concentration in CDCI 3 in 5 mm tubes at 21 °C. The IR spectra were obtained on a Mattson Galaxy 5000 FTIR spectrometer at a resolution of 0.5cm -l using samples at 3% w/v concentration in CHCI3 (ACS spectrophotometric grade).
3. Results and discussion The 13C and ~H chemical shifts of compounds l a - l o are shown in Table 1 along with the corresponding vSCN values. Assignments of ~3C signals were based on substituent chemical shifts (SCS) from the literature [la,9] and this work [10], signal intensity ratios, and 13C-1H 2-D heteronuclear correlations. Assignments of IH signals were based on SCS values from the literature [11] and this work [12], and on computer analysis of A A ' B B ' spectra. 3.1. Substituent chemical shift values f o r the S C N group
The data for thiocyanatobenzene (la) and the chemical shifts for benzene under our operating conditions (6H = 7.32 and 6C = 128.34 ppm) were used to derive the following SCS values (in ppm) for the SCN group. 'H:
--oTSCN
13C: Z scN
0.21; 7SCN --m -3.88;
7 scN
Z SCN 0.08 0.10; _p
1.72; 7 sen
1.89; 7 sen
1.19
A. Bangher et al./Spectrochimica ,4cta Part A 51 (1995) 1703-1713
1705
Table 1 ~H and ~3C N M R chemical shifts~ and IR SCN absorption frequencies b of 4-X-thiocyanatobenzenes ( l a - l o ) X
6SCN
6C-1
6C-2(6)
6C-3(5)
6C-4
6H-2(6)
6H-3(5)
vSCN
NM% NH 2 OMe CI Br 1 Me H~ COMe NO~ OH Et i-Pr SCN NHAc
112.56 112.51 111.56 109.95 109.73 109.63 I 11.00 110.49 109.00 108.03 112.35 111.01 111.03 109.06 110.93
106.58 109.28 113.87 122.78 123.54 124.43 120.58 124.46 130.60 133.41 113.14 120.77 120.80 126.76 118.13
134.45 134.48 133.79 131.47 131.50 131.37 130.96 130.06 128.56 128.75 134.27 130.80 130.83 130.84 132.08
113.16 116.07 115.88 130.48 133.42 139.30 130.70 130.23 129.85 125.13 117.55 129.83 128.44 130.84 120.99
151.70 149.00 161.35 136.23 124.16 95.39 140.25 129.53 137.37 148.07 158.15 146.51 151.13 126.76 139.68
7.32 7.37 7.50 7.47 7.44 7.26 7.43 7.53 7.57 7.67 7.45 7.45 7.46 7.59 7.50
6.76 6.68 6.95 7.41 7.56 7.76 7.24 7.42 7.98 8.29 6.88 7.26 7.29 7.59 7.61
2154.9 2156.3 2158.5 2161.5 2161.9 2162.1 2159.2 2160.7 2163.3 2165.7 2159.0 2159.0 2159.2 2163.1 2159.3
In ppm downfield from TMS. b In cm -~. c 6H-4 = 7.40 ppm.
These uniformly deshielding Zo.m, p values are consistent with the SCN group being an overall electron-withdrawing substituent in the ground state, with a strong - I effect and a weak + M effect (cf. values from Ref. [4a] of 0 " m = 0 . 5 3 , 0"p----0.52, 0 " t = 0 . 5 6 and ~rR = -0.09). Only Z scN is a shielding value, due to the effect of the CN component of the substituent; comparison with the Z cN value of -15.96 [9a] for benzonitrile shows that the S atom attenuates the shielding effect of the CN component on C-I by + 12.08 ppm (76%). The 1H SCS values are comparable with those reported for the SCN group in furan, thiophene and pyrrole systems [13].
3.2. Correlation analyses These were carried out using the MINITAB statistical package [14]. All linear regressions were carried out with a freely fitted intercept, thus giving the same weight to the value of the unsubstituted compound as to the others. The four goodness-of-fit indicators [15] used in this investigation, and (in parentheses) their optimum values, are as follows: s, the standard error of the estimate (0); r and R, the correlation coefficient for simple and multiple linear regression respectively (1); r~ and R 2, the coefficient of determination for simple and multiple linear regression respectively, adjusted for the number of degrees of freedom (100%); F, the test for variance (greater than the critical value). The critical values (e = 0.01) for the correlations described below (Eqs. (1)-(26) and Table 2) were obtained from statistical tables [15], and are as follows: Ft.12 = 9.33; F1A 3 -~ 9.07; F2. 6 = 10.9; F2, 7 = 9.55; ]73,4 ----16.7; F3, 6 = 9.78; F3, 8 - - 7 . 5 9 ; F3, 9 = 6.99. The less precise, but useful, description [16] of correlations being excellent when r (or R) is in the range 1 0.99, satisfactory in the range 0.99-0.95, fair in the range 0.95-0.90, and poor for values <0.90 is also used in the discussion below. The f statistic, used extensively in DSP correlations [1-4], was not used since Slater et al. [17] concluded that it does not offer any information not contained in R,2,.
3.3. Correlations ./'or the S C N carbon shifts The ~3C signal of the SCN group is a weak, sharp one, showing none of the quadrupolar broadening associated with the isomeric NCS group in isothiocyanatobenzenes [18]. It appears at 6110.49 in the parent compound la, and over the range 108,03-112.56 in the substituted compounds l b - l o . These shifts have been correlated with various substituent parameters using SSP and DSP approaches.
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.4. Bangher et al./Spectrochimica .4cta Part .4 51 (1995) 1703-1713
In the SSP approach, d SCN was correlated with Hammett a constants [4a] using Eq. (1), in which the intercept d ° gives the calculated shift of the parent compound In, a can be ap, O'p, + a p or a °, and p is the regression or transmission coefficient [1-4]. d=d°+pa
(1)
The best fit for the substituent set 1 s - I n , as judged by the low value of the standard error of the estimate (s) and the high values of the correlation coefficient (r), the adjusted coefficient of determination (r 2) and the test of variance (F) (see above), involves trp according to Eq. (2). dSCN = 110.57(_+0.03) - 3.23( _+0.08)ap s = 0.12; r = 0.9965; r ,z = 99.2%; F = 1637; n = 14
(2)
Correlations with ap+, O-p°and a p were much lower in quality (e.g. r ,2 = 91.4%, 90.3% and 82.7% respectively). This is consistent with the C atom of the SCN group being neither directly conjugated with, nor completely insulated from, the X substituent. Exner [4a] does not give a value of ap for the OH substituent in non-polar solvents; interpolation, using dSCN for lo and Eq. (2), gives the value as - 0 . 5 5 under our experimental conditions. However, this value is virtually double that ( - 0 . 2 8 ) of the comparable OMe substituent, which indicates that a non-electronic factor such as hydrogen bonding is responsible for the anomalously high d SCN value of compound lo (see Table 1). The goodness-of-fit indicators of Eq. (2) show that d SCN is described accurately (99.2%) by the electronic effects of the 14 substituents X in I n - I n [15]. The negative value of the transmission coefficient indicates that a "reverse" substituent effect [1] applies, i.e. electron-donating substituents have a deshielding effect on ciSCN while electron-withdrawing ones exert a shielding effect. To examine these electronic effects more fully, we factorised ap into its resonance and polar (inductive/field) components using the two DSP Eqs. (3) and (4). d = d °+p:l+pRaR
(3)
d =- d 0 or_ p F a F + p R a ~
(4)
In the well-established Taft model (Eq. (3)), al is the polar substituent parameter, a R is one of four resonance parameters (a °, a n a, tr~ and a +) to allow for variation in the electron demand at the probe site, and Pt and PR are the respective transmission coefficients [1]. The a~ parameter applies when the benzene ring system under investigation is a relatively unperturbed or "neutral" one, tr,~a when it is part of a benzoic acid structure, and a~ and tr,~ when it is particularly electron-rich or electron-deficient respectively. In the later Reynolds model (Eq. (4)), the polar parameter is denoted by a r (to emphasise that it is a field parameter rather than an inductive one), and the resonance parameter is restricted to a,~ [19]. An excellent fit using Taft's model and his redefined set of a~ and a,~ values [20] was obtained for the basis set l a - l i as shown in Eq. (5) and its goodness-of-fit indicators. This correlation was confirmed, and improved slightly, using Reynolds' model, his statistically derived set of a r and a~ values [19], and the larger basis set of substituents l a - l j (including the demanding I) as shown in Eq. (6). d S C N = 110.36(_+0.09) - 2.69(_+0.23) a t - 4.54(_+0.21) a,~
s = 0 . 1 5 ; R = 0.9965; R ] = 9 9 . 1 % ; F = 4 3 6 ; n = 9
(5)
dSCN = 110.45(_+0.09) - 2.94(_+0.22) a F - 4.45(_+0.19) a~ s = 0.13; R = 0.9970; R 2 = 99.2%; F = 547; n = 10
(6)
Fitting the data of the complete set of substituents l a - l o required use of less accurate at and a~ values [4a] for substituents l k - l o and resulted in a poorer correlation (e.g. R ] = 95.5%). Poorer DSP correlations also resulted from the use of (a) a t and a~ values
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A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
[4c] derived from 19F N M R spectra (e.g.
R a- 2 _ 96.6%), (b) a ] A, a+R, and a~ values [4a] (e.g. R ] = 98.4%, 94.4%, and 88.2% respectively) and (c) the revised F and R parameters of Swain et al. [21] (e.g. R 2 = 95.2%). The DSP Eqs. (5) and (6) show, through the negative value of each transmission coefficient, that the reverse substituent effect operates through both the polar and the resonance components of the electronic effect, and that, although the C atom of the SCN group is not directly conjugated with the ring and the substituent X, it is more sensitive to resonance effects than to polar ones as shown by the ratio P R / P t = 2 = 1.69 in Eq. (5). Reverse effects, for both polar and resonance components, have been well documented for many side-chain n-bonded systems, and attributed to 7r-polarisation effects induced by the through-space interactions of the X substituent/ring dipoles with the zr-system [1]. Polar rc-polarisation of the SCN group is illustrated for an electron-withdrawing X substituent in 2, and resonance rc-polarisation for an electron-donating one in the canonical forms 3 and 4 [22].
,5-
,~+
X ~ S - - ~ N 2
3
4
Similar correlations with the tr~ scale and reverse substituent effects have been noted at t3C and 195pt probes in other S-bonded side-chains, e.g. at C-~ in 4-X-C6Ha-SCH--CH 2 ( p z = - 4 . 2 3 ; p R = - 6 . 7 3 ; 2=1.59) [23], at C-I' in 4-X-C6H4 S Ph ( p ~ = - 5 . 0 ; p R = - 1 1 . 7 ; 2=2.34) [24] and at Pt in 4-X-C6H4 S Pt(terpyridine) ( p i = --8.1; pR = --11.8; 2 = 1.46) [25]. Comparison of these transmission coefficients with those in Eq. (5) indicates that, in these systems, the sensitivity of the ~3C probe to reverse substituent effects decreases in the order SPh > SCH=CH2 > SCN; this is possibly due to the effect of increasing bond order and the introduction of the electronegative N atom. Reverse effects ( p ~ = - 2 . 7 ; p R = 1.1; 2=0.41) also occur on the 6CN of 4-X-C6H4-CN systems, but involve the a~ scale since the carbon atom of the CN group, being directly attached to the ring, places a greater electron demand on X [9a]. However, normal substituent effects (pz= 5.89; p g = 7.04; 2 = 1.20) involving the a~ scale are found at the ~3C probe of the isomeric isothiocyanato side-chain in 4-X-C6Ha-N=C=S systems since the C atom is directly conjugated with the ring and X [18]. 3.4. Correlations f o r the ring carbon shifts
Correlations of the shifts of the ring atoms C-I, C-2(6), C-3(5) and C-4 in 1 with the substituent effects of X were examined using SSP, DSP and TSP methods. In the DSP and TSP approaches, Taft's o-~ and a,~ values were used in some cases, e.g. to facilitate comparison of correlations with the extensive Taft-based ones in the literature [1], while Reynolds' o F and a~ values were used in others, e.g. for correlations involving his non-electronic constants (see below). 3.4.1. S S P correlations
In the SSP method, c~C was correlated with the SCS constants of X using Eq. (7), introduced by Lynch [26], in which Z x is the SCS value of X and can be Z x, Z,x, Z,,x or Z x according to the position of X relative to the t3C probe, and b is the transmission coefficient. ~5 = c~° + b Z x
(7)
As shown in Eqs. (8), (9) and (10) respectively, excellent correlations were obtained for gC-1, gC-3(5) and 6C-4 and the complete set of substituents l a - l o , using published [9] and unpublished [10] Z values, but, as in many other investigations [1,24], no correlation
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A. Bangher et aL/Spectrochimica Acta Part A 51 (1995) 1703-1713
was found for ~C-2(6) (r = 0.2983, r~2 = 1.9%, F = 1.28). The latter result is discussed further in the TSP analyses of these shifts (see below). 6C-1 = 124.75(_+0.22)+ 1.50(_+0.04) Z x s = 0.72; r = 0.9955; re = 99.1%; F = 1467; n = 15
(8)
8C-3(5) = 129.99(_+0.15) + 1.02(+0.02) Z x s = 0.53; r = 0.9980; r,,2 = 99.5%; F = 2 9 2 4 ; n = 15
(9)
~C-4 = 129.93(-+0.27) + 1.01(-+0.01) Z x s = 0.88; r = 0.9985; r a2 = 99.7%; F = 4 8 6 7 ; n = 15
(10)
The values of the transmission coefficient (b) for the 6C-3(5) and ~C-4 correlations are very close to unity, indicating that the SCN group has little effect on the additivity of ipso and ortho SCS values, as noted for other groups [26]. However, the b value of 1.50 for the 3C-1 correlation shows that the (ipso) SCN group amplifies para SCS values by 50%. Similar b values for c~C-1 correlations have been reported in other systems with S-bonded side-chains containing multiple bonds, e.g. b = 1.536 for 4-X-C6H4-S-CH=CH2 and b = 1.639 for 4-X-C6H4-S-Ph [24]; however, in the related 4-X-C6H4-NCS [18] and 4-X-C6H4-CN [26] systems the b values are 1.04 and 1.16 respectively. This non-additivity, but proportionality, of para SCS values has been noted for many other 4-X-C6H4-Y systems, with b values ranging from 0.619 for Y = F to 1.66 for Y = N f [26], and has been attributed to C-I responding to donor and aeeeptor substituents X with different sensitivities depending on the nature of the ipso Y group. It has been discussed in terms of the ionisation potentials of the atoms attached to C-1 [26], and also in terms of the effect of X on the shift-charge ratio at C-I [9a]. 3.4.2. D S P
and TSP
correlations
Comprehensive investigations [1] have shown that the DSP Eqs. (3) and (4) are inadequate for the accurate description of the shifts of the ring carbons in a wide range of these 4-X-C6H4-Y systems, but can be amended by addition of a suitable corrective parameter. Two methods have been used successfully for C-I shifts. The first was developed by Bromilow et ai. [9a], and is the non-linear version of Eq. (3), termed the dual substituent parameter-non-linear resonance (DSP-NLR) method, shown in Eq. (11). t~ = 3 0 + p l f f l + pRtr°R/(1 -- ca°R)
(11)
The new parameter e represents the electron demand placed on the para substituent X by Y as measured at the probe site C-1. Eq. (11) thus allows the aR value of X to change in a non-linear manner in response to the nature of Y. When ~C-1 values of the basis set l a - l i were fitted to Eq. (11) using Taft's ~rt and a~ values an excellent correlation was obtained when e had the value of - 0 . 3 4 as shown in Eq. (12). 3C-1 = 124.63(_+0.16) + 7.77(+0.41) az + 27.05(_+0.32) a~/(1 + 0.34a~) s = 0.26; R = 0.9995; R] = 99.9%; F = 4965; n = 9
(12)
The signs and magnitudes of the P I and PR values show that the normal substituent effect is transmitted through both polar and resonance pathways with the latter being dominant (2 = 3.48). Comparison with the corresponding P I and PR values of 4.5 and 21.9 respectively ( 2 = 4 . 8 7 ) for the parent system 4-X-C6H4-H [9a] confirms the enhancement of substituent effect indicated in the SSP Eq. (8), and also shows that the increase in the polar component (73%) is three times larger than that (24%) in the resonance one. Large values of P I and PR are found in other systems containing S-bonded side-chains, e.g. 10.5 and 36.2 respectively in 4-X-C6H4-S-Ph [24a], but not in the
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
1709
related systems 4-X-C6H4-NCS [18] ( P t = 5.64; pR=20.50) and 4-X-C6H4-CN [9a] (Pz = 5.5; PR = 18.5), nor in the extensive series examined by Bromilow et al. [9a]. The e value of - 0 . 3 4 agrees well with the value of - 0 . 3 6 calculated from the at and a $ values of the SCN group [4a] and the equation e = - 0 . 9 a l ~ V ) - 1.6aS(v) [9a], and shows that the SCN group is a moderately strong n-acceptor, e.g. stronger than the isomeric NCS group ( e = - 0 . 2 6 ) [18], similar to SO2Ph ( e = - 0 . 4 ) [24b], but weaker than CN ( e = - 0 . 6 0 ) [9a]. When e was set to zero in Eq. (11), thus reducing it to the normal DSP Eq. (3), a poorer correlation was obtained (s = 0.74; R = 0.9975; R ~2 -_ 99.3%; F = 605). In the second method of dealing with C-1 shifts, Holik [27] uses a linear second-order TSP model of the type shown in Eq. (13), in which the corrective parameter is (a~) 2 and p~ is its transmission coefficient. 6C-1 = 60 + pFCrF + pl~tr oR + p 2R(tr on) 2
(13)
When 6C-1 values of the basis set l a - l j were fitted to Eq. (13) using Reynolds' o"F and ~r~ values another excellent correlation was obtained as shown in Eq. (14). 6C-1 = 124.44(_+0.20) + 8.12(_+0.48) a r + 27.48(--+0.79) a~ -- 9.45(+ 1.71) (a,~) 2 S = 0.28; R = 0.9995; R2, = 99.9%; F = 2772; n = 10
(14)
This correlation closely reproduces the features of the DSP-NLR Eq. (12), e.g. the signs and magnitudes of the polar and resonance parameters as well as their ratio (2 = 3.38). Similarly, comparison with the corresponding PF and P n values of 4.66 and 22.22 respectively in the Holik correlation for the parent system 4-X-C6H4-H [28] shows a 74% increase in a F and a 24% increase in a,~, and the value of the electron demand, defined [27] as the ratio p2R/p R in Eq. (13), is --0.34. Again, omission of the corrective parameter (a~)2, reducing Eq. (13) to DSP Eq. (4), led to a poorer correlation (s = 0.65; R = 0.9980; R,2 = 99.4°/,,; F = 791). Reynolds et al. [19] have shown that correlation of the shifts of non-para carbon atoms in 1,4-disubstituted benzenes, e.g. C-2(6), C-3(5) and C-4 in compounds l a - l o , requires the inclusion of a non-electronic substituent parameter and have developed the TSP model shown in Eq. (15). (~C = 6 0 -~ DFGF-~ pRtT°R "JUpS S
(15)
In this model, the new parameter S represents the non-electronic, short-range component of the substituent effect of X. There are three sets of values, designated /, O and M, which are applied respectively to carbons in the positions ipso, ortho and meta to the substituent X; P s is the corresponding transmission coefficient. When 6C-2(6), 6C-3(5) and 6C-4 values of the basis set l a - l j were fitted to the appropriate form of Eq. (15) using Reynolds' av, a°R and S values, the excellent correlations shown in Eq. (16)-(18) were obtained. 6C-2 = 130.02(-+0.13) - 0.06(_+0.35) a r - - 8.58(_+0.27) a~ + 0.44(_+0.18) M s = 0.19; R = 0.9975; R~ = 99.2%; F = 389; n = 10
(16)
6C-3(5) = 129.96(-+0.33) - 3.15(-+0.82) a r + 23.83(-+0.69) a~ + 1.03(_+0.03) O s = 0.48; R = 0.9990; R] = 99.7%; F = 950; n = 10
(17)
6C-4 = 129.68(-+0.68) + 12.59(_+ 1.71) trF - 40.45(_+ 1.46) ~r~ + 0.98(-+0.02) I s = 1.00; R = 0.9990; R 2 = 99.7%; F = 1019; n = 10
(18)
Omission of the M, O and I parameters, reducing each correlation to a DSP one, resulted in poorer fits, particularly for 6C-3(5) and 6C-4 (e.g. for 6C-2(6), 6C-3(5) and 6C-4, R] = 98.7%, 33.3%, 0.0% and F = 337, 3.25, 0.63 respectively). Examination of Eqs. (14) and (16)-(18) shows that normal resonance effects of X in series 1, indicated by positive PR values, occur at the ortho C-3(5) and para C-I sites, but reverse ones, with negative PR values, occur at the ipso C-4 and meta C-2(6) ~(A)
$1:10-I
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
1710
Table 2 TSP correlations of 6C-2(6) in 4-X~C6H4-Y systems using Eq. (15) with S = M"
Y
PF
PR
PM
)"
S
R
R~, (%)
F
n
H CN NCS SPh SCHCH 2 SCN
1.80 2.36 1.61 -3.44 - 1.19 -0.06
- 1.43 - 2.52 - 1.91 - 12.96 -9.44 -8.58
0.82 0.60 0.81 1.99 1.55 0.44
-0.79 - 1.07 - 1.19 3.71 7.93 143
0.22 0.41 0.14 0.17 0.36 0.19
0.9450 0.8983 0.9844 0.9990 0.9920 0.9975
85.3 73.5 95.8 99.7 97.5 99.2
22 11 93 724 104 389
12 12 13 8 9 10
All symbols as in text.
sites. In contrast, normal polar effects occur at the ipso and para positions, and reverse ones at the ortho and meta positions. For the ipso, ortho and para correlations, these effects and their magnitudes are similar to those noted in other 4-X-C6Ha-Y systems (Y is CH--CH2, OH, CO2Ti(r/-CsHs) 2 (OCOC6H4X)) [19,29]. However, the meta correlation (16) shows a very unusual combination of Pr, PR and PM values and a surprisingly large reverse resonance effect. Reverse resonance effects have been noted before in DSP correlations of meta carbons in other 4-X-C6H4-Y systems [9a], but, to facilitate more accurate comparisons with the results of Eq. (16), the published data [9a,18,23,24] for the parent and some related systems (Y is H, CN, NCS, SPh and SCH--CH2) have been re-evaluated on the TSP basis of Eq. (15), and are presented, along with the data for the SCN group, in Table 2. The values of PF, PR, Pat and 2 for the groups H, CN and NCS are typical of the majority of common groups [30]. However, the S-bonded groups SPh, SCH=CH2 and SCN are atypical in showing negative PF values, large negative PR values (leading to large positive 2 values, particularly for the SCN group), and a wide range of PM values. This predominance of a reverse resonance effect at the meta carbons in compounds with S-bonded side-chains has been attributed [24a] to the ability of sulphur to support canonical forms such as 5, in which X is a + M substituent, and 6, in which it is a - M one. Alternatively, the effect may be due to S-induced ~r-polarisation effects, which are controlled by the ___M effect of X, as shown in 7 and 8. The unusual PF, PR, and p M values in Eq. (16) also explain the failure of Lynch's SSP Eq. (7), using empirical Z x constants derived from monosubstituted benzenes [9,10], to account for shifts at C-2(6) in series 1 since the blend of ar, aR and M in Eq. (16) is very different from that of the parent system 4-X-C6Ha-H (see Table 2) which determines the values of these SCS constants. @
5
@
@
~
6
8+
7
@
8-
8
3,5. Correlations for the ring hydrogen shifts Correlations of the ring hydrogen shifts with the substituent effects of X were examined using Lynch's SSP model (Eq. (7)) and Reynolds' DSP model (Eq. (4)). SSP correlations of 6H-2(6) with Z x, and ~H-3(5) with Z x, using published [11] and revised [12] SCS values and the full substituent set l a - l o , are shown in Eqs. (19) and (20) respectively. c~H-2(6) = 7.50(+0.01) + 0.71(+0.08) Z x s = 0.04; r = 0.9241; r ] = 84.3%; F = 76; n = 15
(19)
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
6H-3(5) = 7.40(+0.01) + 0.94(+0.01) Z x
1711
(20)
s = 0.02; r = 0.9995; r 2 = 99.8%; F = 8722; n = 15 The corresponding DSP correlations, using Reynolds' crF and a~ values and the substituent set l a - l j , are shown in Eqs. (21) and (22) respectively. ~H-2(6) = 7.50(+0.07) + 0.03(+0.17) a F + 0.31(+0.15) a~ S = 0.10; R =0.6519; Rz,= 26.1%; F = 2.59; n = 10
(21)
,~H-3(5) = 7.47(+0.01)+ 0.85(+0.17) GF+ 1.66(+0.15) ~r~ S = 0.10; R = 0.9844; Rz, = 96.1%; F = 111; n = 10
(22)
The SSP correlation of 6 H-3(5) with Z x (Eq. (20)) is excellent and, with a b value near unity, indicates that the (meta) SCN group has little effect on the additivity of ortho SCS values, as noted above for the corresponding correlation of 6 C-3(5). The DSP correlation of 6H-3(5) with aF and a~ (Eq. (22)) shows that the substituent effect is transmitted normally though both polar and resonance pathways, with the latter being the predominant one (2 = 1.95). However, although the DSP correlation is satisfactory, it is significantly inferior to the SSP one, suggesting that, as for the C-3(5) shifts, a TSP model with a third, short-range, non-electronic parameter is necessary to correlate the data with the precision afforded by the empirical SCS parameters which include both electronic and non-electronic components. Regrettably, such parameters are not available for ~i.i shifts. As in the correlations for ~-2(6), the SSP and DSP correlations of 6 H-2(6) with ZXmand with ~F and a~ respectively are considerably lower in quality. The SSP correlation (Eq. (19)) shows only a fair fit, and the b value of 0.71 shows that the (ortho) SCN group has a marked effect on the additivity o f m e t a SCS values, as already noted for 6C-2(6). The DSP correlation (Eq. (21)) suggests that the substituent effect is transmitted normally through both polar and resonance pathways (cf. the reverse substituent effect on 6C-2(6) in Eq. (16)) with a surprisingly high resonance contribution (2 = 10.33), but the very poor quality of the correlation (with F < the critical value of 9.55) calls for considerable caution. Again, a TSP analysis (cf. Eq. (16)) with (unavailable) short-range, non-electronic parameters would be necessary before firm conclusions could be drawn about the relative contributions of the parameters, the modes of transmission of their effects, and any contribution from the anisotropic effect of the SCN group. Similar correlations have been observed in other 4-X-C6H4-Y systems, e.g. Y is SPh, S(O)Ph, SO2Ph [24] and CO2Ti(v/-CsHs)z(OCOC6H4X) [29b]. 3.6. Correlations f o r the S C N stretching frequency
The IR absorption band for the SCN triple bond stretching vibration is a medium-strong, sharp one which permits accurate measurement [6]. In dilute solution in CHCI3, it appears at 2160.7cm -1 in the parent compound la, and over the range 2154.9 2165.7 cm -~ in the compounds l b - l o (see Table 1). These stretching frequencies have been correlated with various substituent parameters using SSP and DSP methods. In SSP correlations of vSCN with Hammett ~ constants (see Eq. (1)), using the substituent set la In, an excellent fit was obtained using ~rp as shown in Eq. (23). vSCN = 2160.1(+0.1) + 7.01(+0.28) ~rp s = 0.41; r = 0.9904; r,] = 97.9%; F = 612; n = 14
(23)
+ Crp, o and a~- were much lower in quality (e.g. r 2 = 93.5, 88.2, and Correlations with ~p, 80.1°/,, respectively). Unlike the corresponding 6SCN values, the vSCN values of compounds 1 (X is OH) and 1 (X is OMe) are very similar (see Table 1), suggesting that the proposed H-bonding effects (see above) are much smaller for vSCN than for 6SCN. When vSCN values were fitted to Reynolds' DSP Eq. (4), using his o-g and a~ constants and the basis set l a - l j , the excellent correlation shown in Eq. (24) was obtained.
1712
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
vSCN = 2160.5(+0.3) + 6.27(+0.61) a t + 9.91(+0.53) a~ (24)
s = 0.37; R = 0.9950; R 2 = 98.7%; F = 350; n = 10 vSCN = 2160.6(+0.2) + 5.93(+0.56) a l + 10.03(+0.50) a~ s = 0.35; R = 0.9960; R 2 = 98.9%; F = 374; n = 9
(25)
A slightly better correlation (Eq. (25)) was obtained using Taft's a~ and a~ values for the substituent set l a - l j, but inferior correlations resulted when (a) a aRA,a I , and a~ values were used (e.g. R a2-_ 98.3%, 96.7%, and 82.6% respectively) and (b) Swain's revised F and R parameters were used (e.g. R~ = 96.3%). The positive values of the transmission coefficients in Eqs. (23)-(25) indicate that vSCN experiences a normal substituent effect which is transmitted through the polar and the resonance pathways. This contrasts sharply with the reverse substituent effect on 6 SCN (see Eqs. (2), (5) and (6)) and is attributed to the effect o f the substituent X on the electron density, and hence the stretching frequency, of the SCN triple bond as shown by the canonical forms 9 and 10.
4-X
CsH4
S~C--~-N
~
9
4-X
CsH4
S
~N
e
10
Thus, an electron-donating substituent X will favour 10, with a CN bond order of two and hence a lower vSCN, whereas an electron-withdrawing substituent will favour 9, with a bond order of three and hence a higher vSCN. Comparison of the transmission coefficients in the vSCN Eqs. (24) and (25) with those in the corresponding J S C N Eqs. (6) and (5) shows that (a) the same blend of polar and resonance effects (2 = 1.62 + 0.09) of X is transmitted to vSCN and JSCN, and (b) vSCN is 2.19(+0.04) times more sensitive than J SCN to each electronic effect. These patterns give rise to an excellent cross-correlation between vSCN and J S C N for the substituent set l a - l n as shown in Eq. (26) vSCN = 2399.6( + 8.6) - 2.17( + 0.08) J SCN s = 0.37; r = 0.9925; r ,2 = 98.3%; F = 774; n = 14
(26)
When the data for 1 (X is OH) were included, the correlation worsened (e.g. ra2 = 92.4%; F = 171), thus confirming the different effects of H-bonding on J S C N and vSCN. Comparison of these vSCN correlations with vCN and vNCS correlations for the related 4-X-C6H4-Y (Y is CN, NCS) systems shows significant differences. Thus, normal substituent effects (al = 6.69; aR = 6.31; 2 = 0.94) occur on vCN but involve the a ] scale since the CN bond is directly conjugated with the substituent X [31], whereas large reverse effects ( a l = -33.48; aR = -31.19; 2 = 0.93), again involving the a~ scale due to conjugation of the NCS bond with X, occur on vNCS [32].
References and notes [1] (a) D.J. Craik, Annu. Rep. NMR Spectrosc., 15 (1983) 1. (b) D.J. Craik and R.T.C. Brownlee, Prog. Phys. Org. Chem., 14 (1983) l, and references cited therein. [2] R.D. Topsom, Prog. Phys. Org. Chem., 16 (1987) 193, and references cited therein. [3] (a) R.D. Topsom, Prog. Phys. Org. Chem., 12 0976) 1. (b) R.W. Taft and R.D. Topsom, Prog. Phys. Org. Chem., 16 (1987) I. (c) W.F. Reynolds, Prog. Phys. Org. Chem., 14 (1983) 165, and references cited therein. [4] (a) O. Exner, Correlation Analysis of Chemical Data, Plenum Press, New York, 1988. (b) J. Shorter, Correlation Analysis of Organic Reactivity, Research Studies Press, John Wiley, Chichester, 1982. (c) J. Shorter and N.B. Chapman (Eds.), Correlation Analysis in Chemistry: Recent Advances, Plenum Press, New York, 1978.
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
1713
[5] (a) M. Giffard, J. Cousseau and G.J. Martin, J. Chem. Soc. Perkin Trans. 2, (1985) 157. (b) L. Testaferri, M. Tingoli, M. Tiecco, D. Chianelli and M. Montanucci, Phosphorus Sulfur, 15 (1983) 263. (c) K. Tamao, T. Kakui and M. Kumada, Tetrahedron Lett., 21 (1980) 111. (d) M. Feigel, H. Kessler and A. Walter, Chem. Ber. 111 (1978) 2947. (e) M. Mishima, M. Fujio, R. Takeda and Y. Tsuno, Mem. Fac. Sci., Kyushu Univ., Ser. C., 11 (1978) 97. (f) R. Radeglia, W. Storek, G. Engelhardt, F. Ritsch, E. Lippmaa, M. M~igi and D. Martin, Org. Magn. Reson., 5 (1973) 419. [6] (a) R.A. Cummins, Aust. J. Chem., 17 (1964) 838. (b) C.N.R. Rao and R. Venkataraghavan, Can. J. Chem., 39 (1961) 1757. (c) G.L. Caldow and H.W. Thompson, Spectrochim. Acta, 13 (1958) 212. [7] R.G. Guy, The Chemistry of Cyanates and Their Thio Derivatives, S. Patai (Ed.), Wiley, Chichester, 1977, p. 819 and references cited therein. [8] E.E. Reid, The Organic Chemistry of Bivalent Sulfur, Vol. 6, E.E. Reid (Ed.). Chemical Publishing, New York, 1965, p. 5 and references cited therein. [9] (a) J. Bromilow, R.T.C. Brownlee, D.J. Craik, M. Sadek and R.W. Taft, J. Org. Chem., 45 (1980) 2429. (b) C.W. Fong, Aust. J. Chem., 33 (1980) 1291. [10] ~3C spectra of PhX at a concentration of Y¼, w/v in CDCI 3 gave, under our conditions, the following values (in ppm) for ZXo.m.p respectively: (a) X is I: -33.98, 9.15, 1.89, -0.89; (b) X is NHAc: 9.63, -8.33, 0.62, -4.04; (c) X is OH: 27.05, -13.02, 1.36, -7.46; (d) X is SCN: see Discussion. [11] J. Beeby, S. Sternhell, T. Hoffmann-Ostenhof, E. Pretsch and W. Simon, Anal. Chem., 45 (1973) 1571. [12] ~H spectra of PhX at a concentration of 3% w/v in CDC13 gave, under our conditions, the following values x p respectively: (a) X is NHAc: 0.18, -0.02, -0.23; (b) X is SCN: see Discussion. (in ppm) for Z .... [13] E. Pretsch, T. Clerc, J. Seibl and W. Simon, Tables of Spectral Data for Structure Determination of Organic Compounds, Springer, Berlin, 1983, p. H285, 290, 300. [14] MINITAB Statistical Software, Release 8, PC Version, Minitab Inc., 3081 Enterprise Drive, State College, PA, 1991. [15] D.L. Harnett, Statistical Methods, 3rd ed., Addison-Wesley, London, 1982. [16] J. Shorter, Ref. 4(c), p. 120. [17] C.D. Slater, C.N. Robinson, R. Bies, D.W. Bryan, K. Chang, A.W. Hill, W.H. Moore, T.G. Otey, M.L. Poppelreiter, J.R. Reisser, G.E. Stablein, V.P. Waddy, W.O. Wilkinson and W.A. Wray, J. Org. Chem., 50 (1985) 4125. [18] R.G. Jones and G. Allen, Org. Magn. Reson., 19 (1982) 196. [19] W.F. Reynolds, A. Gomes, A. Maron, D.W. Maclntyre, A. Tanin, G.K. Hamer and I.R. Peat, Can. J. Chem., 61 (1983) 2376. [20] J. Bromilow, R.T.C. Brownlee, V.O. Lopez and R.W. Taft, J. Org. Chem., 44 (1979) 4766. [21] C.G. Swain, S.H. Unger, N.R. Rosenquist and M,S. Swain, J. Am. Chem. Soc., 105 (1983) 492. [22] J. Bromilow, R.T.C. Brownlee, D.J. Craik, P.R. Fiske, J.E. Rowe and M. Sadek, J. Chem. Soc. Perkin Trans. 2, (1981) 753. [23] W.F. Reynolds and R.A. McClelland, Can. J. Chem., 55 (1977) 536. [24] (a) R. Chandrasekaran, S. Perumal and D.A. Wilson, Magn. Reson. Chem., 25 (1987) 1001. (b) R. Chandrasekaran, S. Perumal and D.A. Wilson, Magn. Reson. Chem., 27 (1989) 360. [25] R.T.C. Brownlee, W.D. McFadyen, M.J. O'Connor, T.J. Rook, I.A.G. Roos and L.P.G. Wakelin, Magn. Resort. Chem., 25 (1987)492. [26] B.M. Lynch, Can. J. Chem., 55 (1977) 541. [27] M. Holik, Magn. Reson. Chem., 30 (1992) 189. [28] This work, using Eq. (13) and data from Ref. [9a]. [29] (a) H.M. Hutton, K.R. Kunz, J.D. Bozek and B.J. Blackburn, Can. J. Chem., 65 (1987) 1316. (b) Y. Dang, H.J. Geise, R. Dommisse, J. Gelan and J. Nouwen, J. Chem. Soc. Perkin Trans. 2, (1990) 1785. [30] R.G. Guy, unpublished work. [31] L.W. Deady, A.R. Katritzky, R.A. Shanks and R.D. Topsom, Spectrochim. Acta Part A, 29 (1973) 115. [32] This work, using Eq. (3) and data from C.M.R. Rao, J. Ramachandran and S. Somasekhara, Current Sci., 27 (1958) 474. Statistical data: s = 4.26; R = 0.9910; R~, = 97,3%; F = 109; n = 7.
7
" *
,
r
I
~
SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 1703 1713
Pseudohalogen chemistry. Part 12. NMR and IR studies of substituent effects in 4-substituted thiocyanatobenzenes A. Bangher, R.G. Guy *, Y. Pichot, J.M. Sillence, C.J. Steel, F.J. Swinbourne, K. Tamiatti School of Natural Sciences, The University of Hertlbrdshire, Hatfield, Hert/brdshire A L 10 9AB, UK
Received 14 November 1994; accepted 21 December 1994
Abstract
~H and ~3C N M R chemicals shifts for 15 4-X-thiocyanatobenzenes have been measured at high dilution in CDCI 3 solution. Substituent chemical shifts for the SCN group were derived from the data for thiocyanatobenzene. Excellent substituent-shift correlations (r > 0.995) were obtained for the SCN carbon using single and dual substituent parameter (SSP and DSP) approaches, and for the ring carbons using SSP, modified DSP, and triple substituent parameter (TSP) approaches. The SCN carbon experiences a reverse substituent effect through both polar and resonance components of the electronic effect of X. Reverse resonance effects were also observed tbr C-2(6) and C-4, and reverse polar effects for C-2(6) and C-3(5). The SCN group causes a 50% enhancement of the normal para SCS values of X at C-I. SSP and DSP substituent-shift correlations were successful for H-3(5) but only fair or poor for H-2(6). IR SCN stretching frequencies were also measured and correlated successfully with substituent parameters using SSP and DSP methods; in contrast to ~SCN, vSCN shows a normal substituent effect.
1. Introduction
There are m a n y literature examples o f the correlation o f nuclear and side-chain IH and 13C N M R chemical shifts in para-substituted benzenes with substituent effects using singleand multi-parameter methods [1]. The effects o f substituents on the I R absorption frequencies of functional groups attached to the benzene ring have been analysed in a similar fashion [2]. These investigations have been instrumental in our understanding o f the nature o f the polar and resonance effects o f substituents on a probe atom or group, their modes o f transmission t h r o u g h the aromatic framework, and their relative contributions [1-3]. In this paper we report the IH and 13C N M R chemical shifts and the S C N I R stretching frequencies of a representative series o f 4-substituted thiocyanatobenzenes ( l a - l o ) . SCN
a:X=H f:X=NO2
b:X=NM~ c:X=NI-~ g:X--COM~ h : X = C 1
d:X=OMe i:X=Br
e:X=M~ |:X=I
k:X=Et
I:X=i-Pr
m:X=NHAe n:X=SCN
o:X=OH
* Corresponding author. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10584 8539(94)01343-1
1704
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
The substituents cover a wide range of electronic effects and include a "basis set" to obtain statistically valid correlations [1,2]. The spectra have been recorded in dilute solution in an inert solvent to minimise solute-solute and solvent-solute interactions [1,2], and the resulting data have been correlated with substituent effects using single, dual and triple substituent parameter (SSP, DSP and TSP) methods [4]. A few earlier reports of the N M R [5] and IR [6] spectra of aromatic thiocyanates were of limited value to this study, since insufficient substituents were studied, concentrations were not recorded, instrument sensitivity was low, or spectral analyses were incomplete.
2. Experimental 2.1. Syntheses
Compounds la, l d - l l , and In were prepared from the corresponding amine by a Sandmeyer reaction using iron(III) thiocyanate as catalyst and were separated from the isothiocyanato co-product by chromatography on a silica gel column [7]. Compounds lb, le and 1o were prepared by reaction of thiocyanogen with the appropriate amine or phenol [7]. Compound lm was prepared by acetylation of le using acetyl chloride. All the compounds have been characterised previously [8]. 2.2. Spectra The 13C N M R spectra were obtained on a Bruker AC 250 spectrometer at a frequency of 62.896 MHz with a pulse width of 2.7 las (45°), using a spectral width of 15.625 kHz and 6 4 K data points to give a maximum attainable resolution of 0.008ppm. The ~H spectra were similarly obtained at a frequency of 250.133 MHz, with a pulse width of 3.0 ~ts (30°), a spectral width of 5.000 KHz and 32 K data points giving a maximum attainable resolution of 0.001 ppm. The spectra were recorded on samples at 3% w/v concentration in CDCI 3 in 5 mm tubes at 21 °C. The IR spectra were obtained on a Mattson Galaxy 5000 FTIR spectrometer at a resolution of 0.5cm -l using samples at 3% w/v concentration in CHCI3 (ACS spectrophotometric grade).
3. Results and discussion The 13C and ~H chemical shifts of compounds l a - l o are shown in Table 1 along with the corresponding vSCN values. Assignments of ~3C signals were based on substituent chemical shifts (SCS) from the literature [la,9] and this work [10], signal intensity ratios, and 13C-1H 2-D heteronuclear correlations. Assignments of IH signals were based on SCS values from the literature [11] and this work [12], and on computer analysis of A A ' B B ' spectra. 3.1. Substituent chemical shift values f o r the S C N group
The data for thiocyanatobenzene (la) and the chemical shifts for benzene under our operating conditions (6H = 7.32 and 6C = 128.34 ppm) were used to derive the following SCS values (in ppm) for the SCN group. 'H:
--oTSCN
13C: Z scN
0.21; 7SCN --m -3.88;
7 scN
Z SCN 0.08 0.10; _p
1.72; 7 sen
1.89; 7 sen
1.19
A. Bangher et al./Spectrochimica ,4cta Part A 51 (1995) 1703-1713
1705
Table 1 ~H and ~3C N M R chemical shifts~ and IR SCN absorption frequencies b of 4-X-thiocyanatobenzenes ( l a - l o ) X
6SCN
6C-1
6C-2(6)
6C-3(5)
6C-4
6H-2(6)
6H-3(5)
vSCN
NM% NH 2 OMe CI Br 1 Me H~ COMe NO~ OH Et i-Pr SCN NHAc
112.56 112.51 111.56 109.95 109.73 109.63 I 11.00 110.49 109.00 108.03 112.35 111.01 111.03 109.06 110.93
106.58 109.28 113.87 122.78 123.54 124.43 120.58 124.46 130.60 133.41 113.14 120.77 120.80 126.76 118.13
134.45 134.48 133.79 131.47 131.50 131.37 130.96 130.06 128.56 128.75 134.27 130.80 130.83 130.84 132.08
113.16 116.07 115.88 130.48 133.42 139.30 130.70 130.23 129.85 125.13 117.55 129.83 128.44 130.84 120.99
151.70 149.00 161.35 136.23 124.16 95.39 140.25 129.53 137.37 148.07 158.15 146.51 151.13 126.76 139.68
7.32 7.37 7.50 7.47 7.44 7.26 7.43 7.53 7.57 7.67 7.45 7.45 7.46 7.59 7.50
6.76 6.68 6.95 7.41 7.56 7.76 7.24 7.42 7.98 8.29 6.88 7.26 7.29 7.59 7.61
2154.9 2156.3 2158.5 2161.5 2161.9 2162.1 2159.2 2160.7 2163.3 2165.7 2159.0 2159.0 2159.2 2163.1 2159.3
In ppm downfield from TMS. b In cm -~. c 6H-4 = 7.40 ppm.
These uniformly deshielding Zo.m, p values are consistent with the SCN group being an overall electron-withdrawing substituent in the ground state, with a strong - I effect and a weak + M effect (cf. values from Ref. [4a] of 0 " m = 0 . 5 3 , 0"p----0.52, 0 " t = 0 . 5 6 and ~rR = -0.09). Only Z scN is a shielding value, due to the effect of the CN component of the substituent; comparison with the Z cN value of -15.96 [9a] for benzonitrile shows that the S atom attenuates the shielding effect of the CN component on C-I by + 12.08 ppm (76%). The 1H SCS values are comparable with those reported for the SCN group in furan, thiophene and pyrrole systems [13].
3.2. Correlation analyses These were carried out using the MINITAB statistical package [14]. All linear regressions were carried out with a freely fitted intercept, thus giving the same weight to the value of the unsubstituted compound as to the others. The four goodness-of-fit indicators [15] used in this investigation, and (in parentheses) their optimum values, are as follows: s, the standard error of the estimate (0); r and R, the correlation coefficient for simple and multiple linear regression respectively (1); r~ and R 2, the coefficient of determination for simple and multiple linear regression respectively, adjusted for the number of degrees of freedom (100%); F, the test for variance (greater than the critical value). The critical values (e = 0.01) for the correlations described below (Eqs. (1)-(26) and Table 2) were obtained from statistical tables [15], and are as follows: Ft.12 = 9.33; F1A 3 -~ 9.07; F2. 6 = 10.9; F2, 7 = 9.55; ]73,4 ----16.7; F3, 6 = 9.78; F3, 8 - - 7 . 5 9 ; F3, 9 = 6.99. The less precise, but useful, description [16] of correlations being excellent when r (or R) is in the range 1 0.99, satisfactory in the range 0.99-0.95, fair in the range 0.95-0.90, and poor for values <0.90 is also used in the discussion below. The f statistic, used extensively in DSP correlations [1-4], was not used since Slater et al. [17] concluded that it does not offer any information not contained in R,2,.
3.3. Correlations ./'or the S C N carbon shifts The ~3C signal of the SCN group is a weak, sharp one, showing none of the quadrupolar broadening associated with the isomeric NCS group in isothiocyanatobenzenes [18]. It appears at 6110.49 in the parent compound la, and over the range 108,03-112.56 in the substituted compounds l b - l o . These shifts have been correlated with various substituent parameters using SSP and DSP approaches.
1706
.4. Bangher et al./Spectrochimica .4cta Part .4 51 (1995) 1703-1713
In the SSP approach, d SCN was correlated with Hammett a constants [4a] using Eq. (1), in which the intercept d ° gives the calculated shift of the parent compound In, a can be ap, O'p, + a p or a °, and p is the regression or transmission coefficient [1-4]. d=d°+pa
(1)
The best fit for the substituent set 1 s - I n , as judged by the low value of the standard error of the estimate (s) and the high values of the correlation coefficient (r), the adjusted coefficient of determination (r 2) and the test of variance (F) (see above), involves trp according to Eq. (2). dSCN = 110.57(_+0.03) - 3.23( _+0.08)ap s = 0.12; r = 0.9965; r ,z = 99.2%; F = 1637; n = 14
(2)
Correlations with ap+, O-p°and a p were much lower in quality (e.g. r ,2 = 91.4%, 90.3% and 82.7% respectively). This is consistent with the C atom of the SCN group being neither directly conjugated with, nor completely insulated from, the X substituent. Exner [4a] does not give a value of ap for the OH substituent in non-polar solvents; interpolation, using dSCN for lo and Eq. (2), gives the value as - 0 . 5 5 under our experimental conditions. However, this value is virtually double that ( - 0 . 2 8 ) of the comparable OMe substituent, which indicates that a non-electronic factor such as hydrogen bonding is responsible for the anomalously high d SCN value of compound lo (see Table 1). The goodness-of-fit indicators of Eq. (2) show that d SCN is described accurately (99.2%) by the electronic effects of the 14 substituents X in I n - I n [15]. The negative value of the transmission coefficient indicates that a "reverse" substituent effect [1] applies, i.e. electron-donating substituents have a deshielding effect on ciSCN while electron-withdrawing ones exert a shielding effect. To examine these electronic effects more fully, we factorised ap into its resonance and polar (inductive/field) components using the two DSP Eqs. (3) and (4). d = d °+p:l+pRaR
(3)
d =- d 0 or_ p F a F + p R a ~
(4)
In the well-established Taft model (Eq. (3)), al is the polar substituent parameter, a R is one of four resonance parameters (a °, a n a, tr~ and a +) to allow for variation in the electron demand at the probe site, and Pt and PR are the respective transmission coefficients [1]. The a~ parameter applies when the benzene ring system under investigation is a relatively unperturbed or "neutral" one, tr,~a when it is part of a benzoic acid structure, and a~ and tr,~ when it is particularly electron-rich or electron-deficient respectively. In the later Reynolds model (Eq. (4)), the polar parameter is denoted by a r (to emphasise that it is a field parameter rather than an inductive one), and the resonance parameter is restricted to a,~ [19]. An excellent fit using Taft's model and his redefined set of a~ and a,~ values [20] was obtained for the basis set l a - l i as shown in Eq. (5) and its goodness-of-fit indicators. This correlation was confirmed, and improved slightly, using Reynolds' model, his statistically derived set of a r and a~ values [19], and the larger basis set of substituents l a - l j (including the demanding I) as shown in Eq. (6). d S C N = 110.36(_+0.09) - 2.69(_+0.23) a t - 4.54(_+0.21) a,~
s = 0 . 1 5 ; R = 0.9965; R ] = 9 9 . 1 % ; F = 4 3 6 ; n = 9
(5)
dSCN = 110.45(_+0.09) - 2.94(_+0.22) a F - 4.45(_+0.19) a~ s = 0.13; R = 0.9970; R 2 = 99.2%; F = 547; n = 10
(6)
Fitting the data of the complete set of substituents l a - l o required use of less accurate at and a~ values [4a] for substituents l k - l o and resulted in a poorer correlation (e.g. R ] = 95.5%). Poorer DSP correlations also resulted from the use of (a) a t and a~ values
1707
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
[4c] derived from 19F N M R spectra (e.g.
R a- 2 _ 96.6%), (b) a ] A, a+R, and a~ values [4a] (e.g. R ] = 98.4%, 94.4%, and 88.2% respectively) and (c) the revised F and R parameters of Swain et al. [21] (e.g. R 2 = 95.2%). The DSP Eqs. (5) and (6) show, through the negative value of each transmission coefficient, that the reverse substituent effect operates through both the polar and the resonance components of the electronic effect, and that, although the C atom of the SCN group is not directly conjugated with the ring and the substituent X, it is more sensitive to resonance effects than to polar ones as shown by the ratio P R / P t = 2 = 1.69 in Eq. (5). Reverse effects, for both polar and resonance components, have been well documented for many side-chain n-bonded systems, and attributed to 7r-polarisation effects induced by the through-space interactions of the X substituent/ring dipoles with the zr-system [1]. Polar rc-polarisation of the SCN group is illustrated for an electron-withdrawing X substituent in 2, and resonance rc-polarisation for an electron-donating one in the canonical forms 3 and 4 [22].
,5-
,~+
X ~ S - - ~ N 2
3
4
Similar correlations with the tr~ scale and reverse substituent effects have been noted at t3C and 195pt probes in other S-bonded side-chains, e.g. at C-~ in 4-X-C6Ha-SCH--CH 2 ( p z = - 4 . 2 3 ; p R = - 6 . 7 3 ; 2=1.59) [23], at C-I' in 4-X-C6H4 S Ph ( p ~ = - 5 . 0 ; p R = - 1 1 . 7 ; 2=2.34) [24] and at Pt in 4-X-C6H4 S Pt(terpyridine) ( p i = --8.1; pR = --11.8; 2 = 1.46) [25]. Comparison of these transmission coefficients with those in Eq. (5) indicates that, in these systems, the sensitivity of the ~3C probe to reverse substituent effects decreases in the order SPh > SCH=CH2 > SCN; this is possibly due to the effect of increasing bond order and the introduction of the electronegative N atom. Reverse effects ( p ~ = - 2 . 7 ; p R = 1.1; 2=0.41) also occur on the 6CN of 4-X-C6H4-CN systems, but involve the a~ scale since the carbon atom of the CN group, being directly attached to the ring, places a greater electron demand on X [9a]. However, normal substituent effects (pz= 5.89; p g = 7.04; 2 = 1.20) involving the a~ scale are found at the ~3C probe of the isomeric isothiocyanato side-chain in 4-X-C6Ha-N=C=S systems since the C atom is directly conjugated with the ring and X [18]. 3.4. Correlations f o r the ring carbon shifts
Correlations of the shifts of the ring atoms C-I, C-2(6), C-3(5) and C-4 in 1 with the substituent effects of X were examined using SSP, DSP and TSP methods. In the DSP and TSP approaches, Taft's o-~ and a,~ values were used in some cases, e.g. to facilitate comparison of correlations with the extensive Taft-based ones in the literature [1], while Reynolds' o F and a~ values were used in others, e.g. for correlations involving his non-electronic constants (see below). 3.4.1. S S P correlations
In the SSP method, c~C was correlated with the SCS constants of X using Eq. (7), introduced by Lynch [26], in which Z x is the SCS value of X and can be Z x, Z,x, Z,,x or Z x according to the position of X relative to the t3C probe, and b is the transmission coefficient. ~5 = c~° + b Z x
(7)
As shown in Eqs. (8), (9) and (10) respectively, excellent correlations were obtained for gC-1, gC-3(5) and 6C-4 and the complete set of substituents l a - l o , using published [9] and unpublished [10] Z values, but, as in many other investigations [1,24], no correlation
1708
A. Bangher et aL/Spectrochimica Acta Part A 51 (1995) 1703-1713
was found for ~C-2(6) (r = 0.2983, r~2 = 1.9%, F = 1.28). The latter result is discussed further in the TSP analyses of these shifts (see below). 6C-1 = 124.75(_+0.22)+ 1.50(_+0.04) Z x s = 0.72; r = 0.9955; re = 99.1%; F = 1467; n = 15
(8)
8C-3(5) = 129.99(_+0.15) + 1.02(+0.02) Z x s = 0.53; r = 0.9980; r,,2 = 99.5%; F = 2 9 2 4 ; n = 15
(9)
~C-4 = 129.93(-+0.27) + 1.01(-+0.01) Z x s = 0.88; r = 0.9985; r a2 = 99.7%; F = 4 8 6 7 ; n = 15
(10)
The values of the transmission coefficient (b) for the 6C-3(5) and ~C-4 correlations are very close to unity, indicating that the SCN group has little effect on the additivity of ipso and ortho SCS values, as noted for other groups [26]. However, the b value of 1.50 for the 3C-1 correlation shows that the (ipso) SCN group amplifies para SCS values by 50%. Similar b values for c~C-1 correlations have been reported in other systems with S-bonded side-chains containing multiple bonds, e.g. b = 1.536 for 4-X-C6H4-S-CH=CH2 and b = 1.639 for 4-X-C6H4-S-Ph [24]; however, in the related 4-X-C6H4-NCS [18] and 4-X-C6H4-CN [26] systems the b values are 1.04 and 1.16 respectively. This non-additivity, but proportionality, of para SCS values has been noted for many other 4-X-C6H4-Y systems, with b values ranging from 0.619 for Y = F to 1.66 for Y = N f [26], and has been attributed to C-I responding to donor and aeeeptor substituents X with different sensitivities depending on the nature of the ipso Y group. It has been discussed in terms of the ionisation potentials of the atoms attached to C-1 [26], and also in terms of the effect of X on the shift-charge ratio at C-I [9a]. 3.4.2. D S P
and TSP
correlations
Comprehensive investigations [1] have shown that the DSP Eqs. (3) and (4) are inadequate for the accurate description of the shifts of the ring carbons in a wide range of these 4-X-C6H4-Y systems, but can be amended by addition of a suitable corrective parameter. Two methods have been used successfully for C-I shifts. The first was developed by Bromilow et ai. [9a], and is the non-linear version of Eq. (3), termed the dual substituent parameter-non-linear resonance (DSP-NLR) method, shown in Eq. (11). t~ = 3 0 + p l f f l + pRtr°R/(1 -- ca°R)
(11)
The new parameter e represents the electron demand placed on the para substituent X by Y as measured at the probe site C-1. Eq. (11) thus allows the aR value of X to change in a non-linear manner in response to the nature of Y. When ~C-1 values of the basis set l a - l i were fitted to Eq. (11) using Taft's ~rt and a~ values an excellent correlation was obtained when e had the value of - 0 . 3 4 as shown in Eq. (12). 3C-1 = 124.63(_+0.16) + 7.77(+0.41) az + 27.05(_+0.32) a~/(1 + 0.34a~) s = 0.26; R = 0.9995; R] = 99.9%; F = 4965; n = 9
(12)
The signs and magnitudes of the P I and PR values show that the normal substituent effect is transmitted through both polar and resonance pathways with the latter being dominant (2 = 3.48). Comparison with the corresponding P I and PR values of 4.5 and 21.9 respectively ( 2 = 4 . 8 7 ) for the parent system 4-X-C6H4-H [9a] confirms the enhancement of substituent effect indicated in the SSP Eq. (8), and also shows that the increase in the polar component (73%) is three times larger than that (24%) in the resonance one. Large values of P I and PR are found in other systems containing S-bonded side-chains, e.g. 10.5 and 36.2 respectively in 4-X-C6H4-S-Ph [24a], but not in the
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
1709
related systems 4-X-C6H4-NCS [18] ( P t = 5.64; pR=20.50) and 4-X-C6H4-CN [9a] (Pz = 5.5; PR = 18.5), nor in the extensive series examined by Bromilow et al. [9a]. The e value of - 0 . 3 4 agrees well with the value of - 0 . 3 6 calculated from the at and a $ values of the SCN group [4a] and the equation e = - 0 . 9 a l ~ V ) - 1.6aS(v) [9a], and shows that the SCN group is a moderately strong n-acceptor, e.g. stronger than the isomeric NCS group ( e = - 0 . 2 6 ) [18], similar to SO2Ph ( e = - 0 . 4 ) [24b], but weaker than CN ( e = - 0 . 6 0 ) [9a]. When e was set to zero in Eq. (11), thus reducing it to the normal DSP Eq. (3), a poorer correlation was obtained (s = 0.74; R = 0.9975; R ~2 -_ 99.3%; F = 605). In the second method of dealing with C-1 shifts, Holik [27] uses a linear second-order TSP model of the type shown in Eq. (13), in which the corrective parameter is (a~) 2 and p~ is its transmission coefficient. 6C-1 = 60 + pFCrF + pl~tr oR + p 2R(tr on) 2
(13)
When 6C-1 values of the basis set l a - l j were fitted to Eq. (13) using Reynolds' o"F and ~r~ values another excellent correlation was obtained as shown in Eq. (14). 6C-1 = 124.44(_+0.20) + 8.12(_+0.48) a r + 27.48(--+0.79) a~ -- 9.45(+ 1.71) (a,~) 2 S = 0.28; R = 0.9995; R2, = 99.9%; F = 2772; n = 10
(14)
This correlation closely reproduces the features of the DSP-NLR Eq. (12), e.g. the signs and magnitudes of the polar and resonance parameters as well as their ratio (2 = 3.38). Similarly, comparison with the corresponding PF and P n values of 4.66 and 22.22 respectively in the Holik correlation for the parent system 4-X-C6H4-H [28] shows a 74% increase in a F and a 24% increase in a,~, and the value of the electron demand, defined [27] as the ratio p2R/p R in Eq. (13), is --0.34. Again, omission of the corrective parameter (a~)2, reducing Eq. (13) to DSP Eq. (4), led to a poorer correlation (s = 0.65; R = 0.9980; R,2 = 99.4°/,,; F = 791). Reynolds et al. [19] have shown that correlation of the shifts of non-para carbon atoms in 1,4-disubstituted benzenes, e.g. C-2(6), C-3(5) and C-4 in compounds l a - l o , requires the inclusion of a non-electronic substituent parameter and have developed the TSP model shown in Eq. (15). (~C = 6 0 -~ DFGF-~ pRtT°R "JUpS S
(15)
In this model, the new parameter S represents the non-electronic, short-range component of the substituent effect of X. There are three sets of values, designated /, O and M, which are applied respectively to carbons in the positions ipso, ortho and meta to the substituent X; P s is the corresponding transmission coefficient. When 6C-2(6), 6C-3(5) and 6C-4 values of the basis set l a - l j were fitted to the appropriate form of Eq. (15) using Reynolds' av, a°R and S values, the excellent correlations shown in Eq. (16)-(18) were obtained. 6C-2 = 130.02(-+0.13) - 0.06(_+0.35) a r - - 8.58(_+0.27) a~ + 0.44(_+0.18) M s = 0.19; R = 0.9975; R~ = 99.2%; F = 389; n = 10
(16)
6C-3(5) = 129.96(-+0.33) - 3.15(-+0.82) a r + 23.83(-+0.69) a~ + 1.03(_+0.03) O s = 0.48; R = 0.9990; R] = 99.7%; F = 950; n = 10
(17)
6C-4 = 129.68(-+0.68) + 12.59(_+ 1.71) trF - 40.45(_+ 1.46) ~r~ + 0.98(-+0.02) I s = 1.00; R = 0.9990; R 2 = 99.7%; F = 1019; n = 10
(18)
Omission of the M, O and I parameters, reducing each correlation to a DSP one, resulted in poorer fits, particularly for 6C-3(5) and 6C-4 (e.g. for 6C-2(6), 6C-3(5) and 6C-4, R] = 98.7%, 33.3%, 0.0% and F = 337, 3.25, 0.63 respectively). Examination of Eqs. (14) and (16)-(18) shows that normal resonance effects of X in series 1, indicated by positive PR values, occur at the ortho C-3(5) and para C-I sites, but reverse ones, with negative PR values, occur at the ipso C-4 and meta C-2(6) ~(A)
$1:10-I
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
1710
Table 2 TSP correlations of 6C-2(6) in 4-X~C6H4-Y systems using Eq. (15) with S = M"
Y
PF
PR
PM
)"
S
R
R~, (%)
F
n
H CN NCS SPh SCHCH 2 SCN
1.80 2.36 1.61 -3.44 - 1.19 -0.06
- 1.43 - 2.52 - 1.91 - 12.96 -9.44 -8.58
0.82 0.60 0.81 1.99 1.55 0.44
-0.79 - 1.07 - 1.19 3.71 7.93 143
0.22 0.41 0.14 0.17 0.36 0.19
0.9450 0.8983 0.9844 0.9990 0.9920 0.9975
85.3 73.5 95.8 99.7 97.5 99.2
22 11 93 724 104 389
12 12 13 8 9 10
All symbols as in text.
sites. In contrast, normal polar effects occur at the ipso and para positions, and reverse ones at the ortho and meta positions. For the ipso, ortho and para correlations, these effects and their magnitudes are similar to those noted in other 4-X-C6Ha-Y systems (Y is CH--CH2, OH, CO2Ti(r/-CsHs) 2 (OCOC6H4X)) [19,29]. However, the meta correlation (16) shows a very unusual combination of Pr, PR and PM values and a surprisingly large reverse resonance effect. Reverse resonance effects have been noted before in DSP correlations of meta carbons in other 4-X-C6H4-Y systems [9a], but, to facilitate more accurate comparisons with the results of Eq. (16), the published data [9a,18,23,24] for the parent and some related systems (Y is H, CN, NCS, SPh and SCH--CH2) have been re-evaluated on the TSP basis of Eq. (15), and are presented, along with the data for the SCN group, in Table 2. The values of PF, PR, Pat and 2 for the groups H, CN and NCS are typical of the majority of common groups [30]. However, the S-bonded groups SPh, SCH=CH2 and SCN are atypical in showing negative PF values, large negative PR values (leading to large positive 2 values, particularly for the SCN group), and a wide range of PM values. This predominance of a reverse resonance effect at the meta carbons in compounds with S-bonded side-chains has been attributed [24a] to the ability of sulphur to support canonical forms such as 5, in which X is a + M substituent, and 6, in which it is a - M one. Alternatively, the effect may be due to S-induced ~r-polarisation effects, which are controlled by the ___M effect of X, as shown in 7 and 8. The unusual PF, PR, and p M values in Eq. (16) also explain the failure of Lynch's SSP Eq. (7), using empirical Z x constants derived from monosubstituted benzenes [9,10], to account for shifts at C-2(6) in series 1 since the blend of ar, aR and M in Eq. (16) is very different from that of the parent system 4-X-C6Ha-H (see Table 2) which determines the values of these SCS constants. @
5
@
@
~
6
8+
7
@
8-
8
3,5. Correlations for the ring hydrogen shifts Correlations of the ring hydrogen shifts with the substituent effects of X were examined using Lynch's SSP model (Eq. (7)) and Reynolds' DSP model (Eq. (4)). SSP correlations of 6H-2(6) with Z x, and ~H-3(5) with Z x, using published [11] and revised [12] SCS values and the full substituent set l a - l o , are shown in Eqs. (19) and (20) respectively. c~H-2(6) = 7.50(+0.01) + 0.71(+0.08) Z x s = 0.04; r = 0.9241; r ] = 84.3%; F = 76; n = 15
(19)
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
6H-3(5) = 7.40(+0.01) + 0.94(+0.01) Z x
1711
(20)
s = 0.02; r = 0.9995; r 2 = 99.8%; F = 8722; n = 15 The corresponding DSP correlations, using Reynolds' crF and a~ values and the substituent set l a - l j , are shown in Eqs. (21) and (22) respectively. ~H-2(6) = 7.50(+0.07) + 0.03(+0.17) a F + 0.31(+0.15) a~ S = 0.10; R =0.6519; Rz,= 26.1%; F = 2.59; n = 10
(21)
,~H-3(5) = 7.47(+0.01)+ 0.85(+0.17) GF+ 1.66(+0.15) ~r~ S = 0.10; R = 0.9844; Rz, = 96.1%; F = 111; n = 10
(22)
The SSP correlation of 6 H-3(5) with Z x (Eq. (20)) is excellent and, with a b value near unity, indicates that the (meta) SCN group has little effect on the additivity of ortho SCS values, as noted above for the corresponding correlation of 6 C-3(5). The DSP correlation of 6H-3(5) with aF and a~ (Eq. (22)) shows that the substituent effect is transmitted normally though both polar and resonance pathways, with the latter being the predominant one (2 = 1.95). However, although the DSP correlation is satisfactory, it is significantly inferior to the SSP one, suggesting that, as for the C-3(5) shifts, a TSP model with a third, short-range, non-electronic parameter is necessary to correlate the data with the precision afforded by the empirical SCS parameters which include both electronic and non-electronic components. Regrettably, such parameters are not available for ~i.i shifts. As in the correlations for ~-2(6), the SSP and DSP correlations of 6 H-2(6) with ZXmand with ~F and a~ respectively are considerably lower in quality. The SSP correlation (Eq. (19)) shows only a fair fit, and the b value of 0.71 shows that the (ortho) SCN group has a marked effect on the additivity o f m e t a SCS values, as already noted for 6C-2(6). The DSP correlation (Eq. (21)) suggests that the substituent effect is transmitted normally through both polar and resonance pathways (cf. the reverse substituent effect on 6C-2(6) in Eq. (16)) with a surprisingly high resonance contribution (2 = 10.33), but the very poor quality of the correlation (with F < the critical value of 9.55) calls for considerable caution. Again, a TSP analysis (cf. Eq. (16)) with (unavailable) short-range, non-electronic parameters would be necessary before firm conclusions could be drawn about the relative contributions of the parameters, the modes of transmission of their effects, and any contribution from the anisotropic effect of the SCN group. Similar correlations have been observed in other 4-X-C6H4-Y systems, e.g. Y is SPh, S(O)Ph, SO2Ph [24] and CO2Ti(v/-CsHs)z(OCOC6H4X) [29b]. 3.6. Correlations f o r the S C N stretching frequency
The IR absorption band for the SCN triple bond stretching vibration is a medium-strong, sharp one which permits accurate measurement [6]. In dilute solution in CHCI3, it appears at 2160.7cm -1 in the parent compound la, and over the range 2154.9 2165.7 cm -~ in the compounds l b - l o (see Table 1). These stretching frequencies have been correlated with various substituent parameters using SSP and DSP methods. In SSP correlations of vSCN with Hammett ~ constants (see Eq. (1)), using the substituent set la In, an excellent fit was obtained using ~rp as shown in Eq. (23). vSCN = 2160.1(+0.1) + 7.01(+0.28) ~rp s = 0.41; r = 0.9904; r,] = 97.9%; F = 612; n = 14
(23)
+ Crp, o and a~- were much lower in quality (e.g. r 2 = 93.5, 88.2, and Correlations with ~p, 80.1°/,, respectively). Unlike the corresponding 6SCN values, the vSCN values of compounds 1 (X is OH) and 1 (X is OMe) are very similar (see Table 1), suggesting that the proposed H-bonding effects (see above) are much smaller for vSCN than for 6SCN. When vSCN values were fitted to Reynolds' DSP Eq. (4), using his o-g and a~ constants and the basis set l a - l j , the excellent correlation shown in Eq. (24) was obtained.
1712
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
vSCN = 2160.5(+0.3) + 6.27(+0.61) a t + 9.91(+0.53) a~ (24)
s = 0.37; R = 0.9950; R 2 = 98.7%; F = 350; n = 10 vSCN = 2160.6(+0.2) + 5.93(+0.56) a l + 10.03(+0.50) a~ s = 0.35; R = 0.9960; R 2 = 98.9%; F = 374; n = 9
(25)
A slightly better correlation (Eq. (25)) was obtained using Taft's a~ and a~ values for the substituent set l a - l j, but inferior correlations resulted when (a) a aRA,a I , and a~ values were used (e.g. R a2-_ 98.3%, 96.7%, and 82.6% respectively) and (b) Swain's revised F and R parameters were used (e.g. R~ = 96.3%). The positive values of the transmission coefficients in Eqs. (23)-(25) indicate that vSCN experiences a normal substituent effect which is transmitted through the polar and the resonance pathways. This contrasts sharply with the reverse substituent effect on 6 SCN (see Eqs. (2), (5) and (6)) and is attributed to the effect o f the substituent X on the electron density, and hence the stretching frequency, of the SCN triple bond as shown by the canonical forms 9 and 10.
4-X
CsH4
S~C--~-N
~
9
4-X
CsH4
S
~N
e
10
Thus, an electron-donating substituent X will favour 10, with a CN bond order of two and hence a lower vSCN, whereas an electron-withdrawing substituent will favour 9, with a bond order of three and hence a higher vSCN. Comparison of the transmission coefficients in the vSCN Eqs. (24) and (25) with those in the corresponding J S C N Eqs. (6) and (5) shows that (a) the same blend of polar and resonance effects (2 = 1.62 + 0.09) of X is transmitted to vSCN and JSCN, and (b) vSCN is 2.19(+0.04) times more sensitive than J SCN to each electronic effect. These patterns give rise to an excellent cross-correlation between vSCN and J S C N for the substituent set l a - l n as shown in Eq. (26) vSCN = 2399.6( + 8.6) - 2.17( + 0.08) J SCN s = 0.37; r = 0.9925; r ,2 = 98.3%; F = 774; n = 14
(26)
When the data for 1 (X is OH) were included, the correlation worsened (e.g. ra2 = 92.4%; F = 171), thus confirming the different effects of H-bonding on J S C N and vSCN. Comparison of these vSCN correlations with vCN and vNCS correlations for the related 4-X-C6H4-Y (Y is CN, NCS) systems shows significant differences. Thus, normal substituent effects (al = 6.69; aR = 6.31; 2 = 0.94) occur on vCN but involve the a ] scale since the CN bond is directly conjugated with the substituent X [31], whereas large reverse effects ( a l = -33.48; aR = -31.19; 2 = 0.93), again involving the a~ scale due to conjugation of the NCS bond with X, occur on vNCS [32].
References and notes [1] (a) D.J. Craik, Annu. Rep. NMR Spectrosc., 15 (1983) 1. (b) D.J. Craik and R.T.C. Brownlee, Prog. Phys. Org. Chem., 14 (1983) l, and references cited therein. [2] R.D. Topsom, Prog. Phys. Org. Chem., 16 (1987) 193, and references cited therein. [3] (a) R.D. Topsom, Prog. Phys. Org. Chem., 12 0976) 1. (b) R.W. Taft and R.D. Topsom, Prog. Phys. Org. Chem., 16 (1987) I. (c) W.F. Reynolds, Prog. Phys. Org. Chem., 14 (1983) 165, and references cited therein. [4] (a) O. Exner, Correlation Analysis of Chemical Data, Plenum Press, New York, 1988. (b) J. Shorter, Correlation Analysis of Organic Reactivity, Research Studies Press, John Wiley, Chichester, 1982. (c) J. Shorter and N.B. Chapman (Eds.), Correlation Analysis in Chemistry: Recent Advances, Plenum Press, New York, 1978.
A. Bangher et al./Spectrochimica Acta Part A 51 (1995) 1703-1713
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[5] (a) M. Giffard, J. Cousseau and G.J. Martin, J. Chem. Soc. Perkin Trans. 2, (1985) 157. (b) L. Testaferri, M. Tingoli, M. Tiecco, D. Chianelli and M. Montanucci, Phosphorus Sulfur, 15 (1983) 263. (c) K. Tamao, T. Kakui and M. Kumada, Tetrahedron Lett., 21 (1980) 111. (d) M. Feigel, H. Kessler and A. Walter, Chem. Ber. 111 (1978) 2947. (e) M. Mishima, M. Fujio, R. Takeda and Y. Tsuno, Mem. Fac. Sci., Kyushu Univ., Ser. C., 11 (1978) 97. (f) R. Radeglia, W. Storek, G. Engelhardt, F. Ritsch, E. Lippmaa, M. M~igi and D. Martin, Org. Magn. Reson., 5 (1973) 419. [6] (a) R.A. Cummins, Aust. J. Chem., 17 (1964) 838. (b) C.N.R. Rao and R. Venkataraghavan, Can. J. Chem., 39 (1961) 1757. (c) G.L. Caldow and H.W. Thompson, Spectrochim. Acta, 13 (1958) 212. [7] R.G. Guy, The Chemistry of Cyanates and Their Thio Derivatives, S. Patai (Ed.), Wiley, Chichester, 1977, p. 819 and references cited therein. [8] E.E. Reid, The Organic Chemistry of Bivalent Sulfur, Vol. 6, E.E. Reid (Ed.). Chemical Publishing, New York, 1965, p. 5 and references cited therein. [9] (a) J. Bromilow, R.T.C. Brownlee, D.J. Craik, M. Sadek and R.W. Taft, J. Org. Chem., 45 (1980) 2429. (b) C.W. Fong, Aust. J. Chem., 33 (1980) 1291. [10] ~3C spectra of PhX at a concentration of Y¼, w/v in CDCI 3 gave, under our conditions, the following values (in ppm) for ZXo.m.p respectively: (a) X is I: -33.98, 9.15, 1.89, -0.89; (b) X is NHAc: 9.63, -8.33, 0.62, -4.04; (c) X is OH: 27.05, -13.02, 1.36, -7.46; (d) X is SCN: see Discussion. [11] J. Beeby, S. Sternhell, T. Hoffmann-Ostenhof, E. Pretsch and W. Simon, Anal. Chem., 45 (1973) 1571. [12] ~H spectra of PhX at a concentration of 3% w/v in CDC13 gave, under our conditions, the following values x p respectively: (a) X is NHAc: 0.18, -0.02, -0.23; (b) X is SCN: see Discussion. (in ppm) for Z .... [13] E. Pretsch, T. Clerc, J. Seibl and W. Simon, Tables of Spectral Data for Structure Determination of Organic Compounds, Springer, Berlin, 1983, p. H285, 290, 300. [14] MINITAB Statistical Software, Release 8, PC Version, Minitab Inc., 3081 Enterprise Drive, State College, PA, 1991. [15] D.L. Harnett, Statistical Methods, 3rd ed., Addison-Wesley, London, 1982. [16] J. Shorter, Ref. 4(c), p. 120. [17] C.D. Slater, C.N. Robinson, R. Bies, D.W. Bryan, K. Chang, A.W. Hill, W.H. Moore, T.G. Otey, M.L. Poppelreiter, J.R. Reisser, G.E. Stablein, V.P. Waddy, W.O. Wilkinson and W.A. Wray, J. Org. Chem., 50 (1985) 4125. [18] R.G. Jones and G. Allen, Org. Magn. Reson., 19 (1982) 196. [19] W.F. Reynolds, A. Gomes, A. Maron, D.W. Maclntyre, A. Tanin, G.K. Hamer and I.R. Peat, Can. J. Chem., 61 (1983) 2376. [20] J. Bromilow, R.T.C. Brownlee, V.O. Lopez and R.W. Taft, J. Org. Chem., 44 (1979) 4766. [21] C.G. Swain, S.H. Unger, N.R. Rosenquist and M,S. Swain, J. Am. Chem. Soc., 105 (1983) 492. [22] J. Bromilow, R.T.C. Brownlee, D.J. Craik, P.R. Fiske, J.E. Rowe and M. Sadek, J. Chem. Soc. Perkin Trans. 2, (1981) 753. [23] W.F. Reynolds and R.A. McClelland, Can. J. Chem., 55 (1977) 536. [24] (a) R. Chandrasekaran, S. Perumal and D.A. Wilson, Magn. Reson. Chem., 25 (1987) 1001. (b) R. Chandrasekaran, S. Perumal and D.A. Wilson, Magn. Reson. Chem., 27 (1989) 360. [25] R.T.C. Brownlee, W.D. McFadyen, M.J. O'Connor, T.J. Rook, I.A.G. Roos and L.P.G. Wakelin, Magn. Resort. Chem., 25 (1987)492. [26] B.M. Lynch, Can. J. Chem., 55 (1977) 541. [27] M. Holik, Magn. Reson. Chem., 30 (1992) 189. [28] This work, using Eq. (13) and data from Ref. [9a]. [29] (a) H.M. Hutton, K.R. Kunz, J.D. Bozek and B.J. Blackburn, Can. J. Chem., 65 (1987) 1316. (b) Y. Dang, H.J. Geise, R. Dommisse, J. Gelan and J. Nouwen, J. Chem. Soc. Perkin Trans. 2, (1990) 1785. [30] R.G. Guy, unpublished work. [31] L.W. Deady, A.R. Katritzky, R.A. Shanks and R.D. Topsom, Spectrochim. Acta Part A, 29 (1973) 115. [32] This work, using Eq. (3) and data from C.M.R. Rao, J. Ramachandran and S. Somasekhara, Current Sci., 27 (1958) 474. Statistical data: s = 4.26; R = 0.9910; R~, = 97,3%; F = 109; n = 7.