Alkyl Carbon-Hydrogen Vibrations in physics

7/28/2019 Alkyl Carbon-Hydrogen Vibrations in physics

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C H A P T E R 3

A lkyl Carbon-H ydrogen VibrationsSu m m ar yOther n-Alkane Vibrations1,2-EpoxyalkanesSodium Dimethylphosphonate (CH3)2P(O)2NaMethyhhiomethyl Mercury, Dimethylmercury (4) ,and Methyhhiochloroformate (5)CycloalkanesReferencesFigures TablesFigure 3-1 36 (31)Figure 3-2 37 (31)Figure 3-3 38 (32)Figure 3-4 39 (32)Figure 3-5 39 (32)Figure 3-6 40 (32)Figure 3-7 40 (32)

Table 3-1Table 3-2Table 3-2aTable 3-3Table 3-4Table 3-5Table 3-6Table 3-7Table 3-8Table 3-9Table 3-10Table 3-11Table 3-12Table 3-13

*Numbers in parentheses indicate in- text page reference.

41 (31)42 (32)43 (32)44 (32)45 (32)45 (34)46 (34)47 (34)47 (35)48 (35)49 (35)50 (35)52 (35)53 (35)

32343434353535

T h i s ch ap te r d i scu sse s a lk y l c a r b o n - h y d r o g en m o lecu la r v ib r a t io n s an d , i n so m e ca se s , l o o k s a th o w th e se m o lecu la r v ib r a t io n s a r e a f f ec t ed b y th e i r su r r o u n d in g ch em ica l en v i r o n m en t .Ho wev e r , i n so m e ca ses t h e a lk y l c a r b o n - h y d r o g e n v ib r a t io n s w i l l b e i n c lu d ed in t h e sec t io nth a t d i scu sse s t h e i r m o s t d i s t i n g u i sh in g m o lecu la r v ib r a t io n s .

The ser ies o f n -a lkanes , CnH2n+2wer e p r ep a r ed a s 0 .5 w t . % so lu t io n s in CCI 4, CDC1 3 , an d


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3 2 Alkyl Carbon-Hydrogen VibrationsThe vsym. CH3 mode for th i s ser ies o f n -a lkanes shows tha t i t genera l ly decreases by

ap p ro x i ma te ly 0 . 8 c m - 1 i n C C I 4 so l u t io n an d ap p ro x ima te ly 1 .2 cm -1 i n CDC1 3 so lu t i o nprogress ing in the ser ies C 5 H 1 2 t o C 1 8 H 3 8 in go ing from so lu t ion in C C I 4 t o solut ion in CDC13.

Table 3.2 l ists the IR absorbance data for C 5 H 1 2 t o C 1 8 H 3 8 for the vasym. CH3 and vsym. CH3modes, Fig . 3 .3 shows a p lo t o f (vasym. CH3)/(vasym.CH2) vs A(vsym. CH3)/A(vsym. CH2) inC C 1 4 so lu t ion for C5H12 t o C 1 8 H 3 8 , and Fig . 3 .4 shows a p lo t o f (vsym. CH3)/(vsym. CH2) vsA(v asy m. CH3)/A(vasym. CH2) in C C I 4 so lu t ion for C 5 H 1 2 t o C 1 8 H 3 8 . Both p lo ts show anessen t ia l ly l inear re la t ionsh ip . The s l igh t dev ia t ion from l inear i ty i s most l ike ly due to over-lapping in terferences in the measurement of these peak he ight absorbances .

Table 3.2a l ists absorbance rat ios; al l of these absorbance rat ios for the vCH3an d vCH2m o d e sgenera l ly decrease progress ing in the ser ies C 5 H 1 2 t o C 1 8 H 3 8 .

Table 3.3 l ists IR data for vasym. CH2 and vsym. CH2 for the n-a lkane at 0.5 wt. % in CC14,CDC13, and 54 .6 mo l % CDC13/CC14 so lu t ions . Figure 3 .5 s how s p lo ts o f vasym. CH2 vs vsym.CH2 in each of the so lvent sys tems, an d Fig . 3 .6 shows p lo ts o f vasym. CH3 vs vsym. CH3 in a l lth ree so lvent sys tems. These p lo ts show tha t these re la t ionsh ips a re no t l inear over the en t i re n -a lkane ser ies. Th e p lo ts do p o in t ou t in genera l tha t as vasym. CH2 decreases in f requency , vsym.CH2 a lso decreases in f requ ency progress ing in the ser ies C5H~2 to C18H38, and tha t the vasym.CH3 and vsym. CH3 frequencies show the same t rend .

A s tudy of Tables 3 .1 and 3 .3 show some in teres t ing t rends in CC14. CDC13, or 54 .6 mol %solutio ns p rogr essin g in the series C5H12 to C18H38. The vasym. CH3 frequ ency d ecreas e in goingfrom so lu t ion in CC14 to so lu t ion in CDC 13 i s smal l ( ~ 0 .1 cm -1) whi le for vsym. CH3 thefrequency decrease i s mo re in CDC13 so lu t ion (1 .2 cm -1) than in CC14 so lu t ion (0 . 7cm -1) . Inaddit ion, the freq uen cy difference for vasym. CH3 in CC14 and in CDC13 increas es from 0.11 to0 .22 cm -1 and for vsym. CH2 in CC14 and in CDC13 decreases f rom 0 .45 to 0 .85 progress ing inthe series C 5 H 1 2 t o C 1 8 H 3 8 . The vasym. C H 2 f requency decrease in go ing from so lu t ion in C C 1 4to solut ion is small (0.1 cm -1) wh ile for vsym . CH2 the f requency decrease i s l a rger in go ingfrom so lu t ion in C C 1 4 t o a so lu t ion in CDC13 (0 .6 cm -1) p rogress ing in the ser ies C 5 H 1 2 t oC 1 8 H 3 8 . Moreover , these da ta show tha t the vsym. CH2 mode changes in f requency by a fac tor o fapprox imate ly 5 t imes m ore than vasym. CH2, vasym. CH3, and vsym. C H3 . In addi t ion , thevasym. CH2 frequency increases in f requency whi le the vsym. CH2 f requency decreases infrequency in go ing from so lu t ion in CC1 4 t o CDC13. In genera l these las t two t rends genera l lydecrease progress ing in the ser ies C 5 H 1 2 t o C 1 8 H 3 8 .

Table 3 .4 l i s t s the f requency d i fference be tween vasym. CH3 and vsym. CH3 and be tweenvasym. CH2 and vsym. CH2 in the th ree so lvent sys tems. These da ta show tha t the f requencysepara t ion i s much la rger for the two vCH3vibra t ions ( ~ 85 -1) than for the two vCH2vibra t ions( '-~ 69 cm -1) . Figure 3 .7 show s p lo ts o f vasym. CH 3--vs ym . C H 3 v s vasym. C H 2 - - v s y m . CH2,which c lear ly shows the behavior o f the f requency separa t ion of the vCH3a n d vCH2vibra t ions inthe th ree so lvent sys tems.


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Variables in Data Interpretation 3 3v asy m . CH2 o ccu r s i n t h e reg io n 2 9 2 6 . 6 1 - 2 9 2 7 .7 3 cm - 1 in CC14 a n d i n t h e r e g i o n 2 9 2 6 . 9 2 -2 9 2 8 .1 4 cm - 1 in CDC1 3 , an d v sy m . CH2 o ccu r s i n t h e r eg io n 2 8 5 4 .5 9 - 2 8 6 1 .8 2 cm -1 in CC14 a n din th e r eg io n 2 8 5 4 .5 5 - 2 8 6 1 .2 1 cm -1 in CDC 13. In add i t ion , these four v ib ra t io ns dec rease inf r eq u en cy p r o g r e ss in g in t h e se r i e s C5H12 t h r o u g h C18H38.

T h e n - a l k a n e s a r e n o n p o l a r m o l e c u l e s , a n d o n e w o u l d e x p e c t t h a t t h e r e w o u l d b e m i n i m a lso lu t e - so lv en t i n t e r ac t io n b e tw een n - a lk an e m o lecu le s an d so lv en t m o lecu le s su c h a s CC1 4 an dCDC1 3. T h e n - a lk an es in g o in g f r o m so lu t io n CC1 4 to so lu t io n in C DC1 3 so lu t io n sh o w af reque ncy inc rease o f 0 .11 to 0 .22 cm -1 fo r vasym. CH 3 an d f o r v sy m . CH 3 i t de c re a s e s b y - 0 . 4 5t o - 0 . 8 5 cm -1, wi th d ec r ea se s f o r v sy m . CH 3 p r o g r e ss in g in t h e o r d e r C5H12 to C18H38. Inaddi t ion , the vasym. CH2 f requency d i f fe rence i s 0 .41 to 0 .23 cm -1 an d f o r v sy m . CH2 i s - 0 .6 1to - 0 .0 4 cm - 1 . Ag a in th e v a sy m . CH2 m o d e in c r ea se s i n f r eq u en cy an d th e v sy m . CH2 m o d ed ec r ea se s i n f r eq u en cy in g o in g f r o m so lu t io n in CC14 to so lu t io n in CDC1 3 . T h u s , b o th v a sy m .CH3 an d v sy m . CH2 in c r ea se in f r eq u en cy an d b o th v sy m . CH3 an d v sy m . CH2 d ec r ea se inf r eq u en cy in g o in g f r o m so lu t io n in CC14 to so lu t io n in CDC1 3 a t 0 .5 w t . % so lu t io n s . T h ese d a t aco n f i r m th a t t h e e f f ec t s of th e se so lv en t s a r e m in o r i n t h e se f o u r m o lec u la r s t r e t ch in g v ib r a t io n s .I t i s n o te wo r th y th a t t h e v sy m . CH2 v ib r a t io n sh i f t s p r o g r e ss iv e ly to l o wer f r eq u en c y in t h e o r d e rC5H12 to C18H38, an d th a t i t d ec r ea se s i n f r eq u en c y b y a f ac to r o f a t l e a st 7 t im es m o r e th an th evsym. CH3 v ibra t ion .

Ap p a r en t ly , t h e n - a lk an e p r o to n s f o r m weak in t e r m o lecu la r h y d r o g en b o n d s wi th th e f r ee p a i ro f e lec t rons on the C1 a toms of the CC14 a n d / o r C D C 1 3 s o l v e n t s y s te m , a n d a n e x p l a n a t i o n i sn eed ed to d e t e r m in e th e f r eq u en cy b eh av io r o f t h e se f o u r m o lecu la r v ib r a t io n s in g o in g f r o mso lu t io n in CC14 to so lu t io n in CDC13. As these vasym. CH3, vasym. CH2, vsym. CH3, an d v sy m .CH2 m o d es v ib r a t e , t h e p r o to n s o b ta in a weak p o s i t i v e ch a r g e an d th e ca r b o n a to m o b ta in s aweak n eg a t iv e ch a r g e . T h i s i s t h e so - ca l l ed d ip o le m o m en t ch an g e d u r in g th e se m o lecu la rv ib r a t io n s . T h e r e f o r e , t h e n - a lk an e p r o to n s wo u ld f o r m weak in t e r m o lecu la r h y d r o g en b o n d swi th th e f r ee p a i r o f e l ec t r o n s o n th e C1 a to m s o f th e CC1 4 an d /o r CDC1 3 so lv en t sy s t em . T h e C1a to m s o f CC14 wo u ld b e ex p ec ted to b e m o r e b a s i c t h a n th o se f o r CDC1 3 d u e to t h e f ac t th a t t h eD a to m a t t r ac t s e l ec tr o n s f r o m th e C1 a to m s . I n ad d i t i o n , t h e r e i s i n t e r m o lecu la r b o n d in gbet we en D a nd C1 suc h as CC13D: C1CC12D : C1CC13. Therefore , one w oul d expe ct a s t rong erC - H : C1 b o n d to b e f o r m ed b e twee n th e p r o to n s in n - a lk an es an d th e C1 a to m s in CC1 4 th an f o rC1 a to m s in CDC1 3 . Hy d r o g en b o n d in g a l so weak en s th e O- H o r C- H b o n d , an d th e v ib r a t io nv O H : X o r v C - H : X i s e x p e c t e d to d ec r e a se i n f r e q u e n c y ~ t h i s i s w h a t w e n o t e d in t h e vs y m .CH 3 an d v sy m . CH2 m o d es . Ho wev e r , t h e o p p o s i t e was o b se r v ed f o r v a sy m . CH 3 an d v asy m .CH2 , wh e r e b o th m o d es in c r ea sed in f r eq u en cy in g o in g f r o m so lu t io n in CC1 4 to so lu t io n inCDC13 . T h i s f r eq u en cy in c r ea se f o r t h e v a sy m , m o d es n eed s a n ex p lan a t io n . Becau se th e v a sy m .CH 3 an d v asy m . CH2 m o d es in c r ea se in f r eq u en cy , i t r eq u i r e s m o r e en e r g y f o r t h e se two m o d esto v ib r a te i n g o in g f r o m so lu t io n in CC1 4 to so lu t io n in CDC13. T h e two CH 3 g r o u p s a r e i so la t edb y ( CH2) ~ g r o u p s , an d a s t h e C1 a to m s in CD C1 3 a r e weak e r b a se s t h a n th e C1 a to m s in CC1 4 aw e a k e r C - H . . . C 1 C D C 1 2 b o n d i s e x p e c t e d . C o n s e q u e n t l y , t h e v a s y m . CH3 an d v asy m . CH2


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34 Alkyl Carbon-Hydrogen VibrationsOn the o the r hand the re a r e (CH2) , un i t s p res en t in the n -a lkane s e r i e s wh ich a re capab le o f

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in e i the r CC14 o r CDC13 s o lu t ion . I t is no ted tha t v s ym. CH3 inc reas ing ly dec reas es in f r equ encyprog ress in g in the ser ies C 5H12 to C18H38 in going f rom s olu t ion in CC14 to solut io n in CDC13.Th is ind ica tes tha t the C-H:C1CDC13 bond s t r eng th i s inc reas ed as n i s inc reas ed fo rCH3(CH2)nCH3.The induc t ive e f fec t o f add i t iona l CH2 g roups a pparen t ly we aken s the CH3bonds , caus ing vs ym. CH2 to dec reas e in f requency as the num be r o f CH2 g roup s a r e inc reas ed inthe n -a lkane . As the number o f CH2 g roups a r e inc reas ed , the dec reas e in vs ym. CH2 fo r n -a lkanes dec reas es p rogres s ing in the s e r ie s C5H12 to C10H22 and i s r ela t ive ly con s tan t f romC9H20 to C18H38 Th is s ugg es t s tha t the e f f ec t o f the n um ber o f CH2 g roups fo rmingin te rmolecu le hydro gen bo nds w i th a CDC13 cha in i s min imiz ed a f t e r e igh t CH2 g rou ps a r ep res en t in tha t the e f fec t o f the nu m ber o f (CH2)C1CDC13 in te rmolecu la r hy drog en bo ndsform ed betw een the n-alkane and CDC13 is m inim ized in the ser ies Cl lH24 to C18H38 I t is a lsopos s ib le tha t the C1 a toms in CDC13 a re c lose r in s pace to the C -H bon ds com pared to tha t fo rCC14, and th is fact wo uld also contr ib ute to lower vsym. CH2 f requencies .

In the ser ies C5H12 to C18H38 there is a com para t ively large chan ge in the vsym. (CH 2) , m ode.There i s a dec rease o f 7 . 23 cm -1 CC14 and 6 . 6 6cm -1 in CDC13 . Th is i s a t t r ibu ted to theinc reas ing number o f CH2 g roups s t r e tch ing in -phas e p rogres s ing in the n -a lkane s e r i e s .

The s moo th co r r e la t ion o f the abs o rbance va lues o f the CH2 and CH3 g roups as the r a t io o fthe CH3 g roups to CH2 g roup s dec reas es i s ju s t wha t i s p red ic ted . T here i s apparen t ly nos ign i fi can t d i ff e r ence in the d ipo le m om ent s o f e i the r CH2 o r CH3 s t re tches p rogres s ing in the n -alkane ser ies .

O T H E R n - A L K A N E V I B R A T I O N SThe CH2 bend as ym. CH3 bend , and the s ym. CH 3 ben d occur s ne a r 1467 , 1458 , and1378.5 cm -1 in CC14 solu t ion (Table 3 .5) .

1 , 2 - E P O X Y A L K A N E STable 3 .6 l is ts IR vap or-ph ase data for the a lkyl (R) v ibra t ions o f 1 ,2-epoxy alkan es (2) . Thevas ym. CH3 mo de o ccur s in the r eg ion 295 3-29 72 cm -1 , the vas ym. CH2 mod e in the r eg ion2 9 2 0 - 2 9 3 5 c m - 1 , t h e v s y m . C H 2 m o d e i n t h e r e g i o n 2 8 7 0 - 2 9 3 2 c m - 1 , a n d t h e v s y m . C H 3


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Variables in Data Interpretation 3 5( CH3 )2 b en d in g m o d es a r e a s s ig n ed a t 1 4 2 8 an d 1 4 1 3 cm - 1 an d th e sy m . ( CH3 ) 2 b en d in g m o d esa r e a s s ig n ed a t 1 2 9 3 an d 1 2 8 4 cm - 1 .

M E T H Y L T H I O M E T H Y L M E R C U R Y , D I M E T H Y L M E R C U R Y (4), A N DM E T H Y L T H I O C H L O R O F O R M A T E ( 5 )T ab le 3 .8 l i s t s a s s ig n m en t s f o r t h e CH3 - Hg an d CH3 - S g r o u p s f o r t h e p r eced in g 3 co m p o u n d s .T h ese a s s ig n m en t s sh o u ld a id t h e r ead e r i n a s s ig n in g v ib r a t io n s f o r t h e se CH3 g r o u p s in o th e rc o m p o u n d s .

C Y C L O A L K A N E ST ab le 3 .9 l i s ts I R v ap o r - p h ase d a t a f o r cy c lo a lk an es ( 6 ). R am a n d a t a f o r th e r i n g b r ea th in g an dr in g d e f o r m a t io n m o d es a r e a l so p r e sen ted f o r cy c lo b u tan e an d cy c lo p en tan e . T h e v a sy m . CH2m o d e o c c u r s i n t h e r e g i o n 2 9 3 0 - 3 1 0 0 c m - 1 . T h e v s y m . C H 2 m o d e o c c u r s i n t h e r e g i on 2 8 8 0 -3 0 2 0 c m - 1 . B o t h v C H 2 v ib r a t io n s d ec r ea se in f r eq u en cy a s t h e r i n g b eco m es l a r g e r . T h i s i s t h er e su l t o f l e s se r r in g s t r a in w i th in c r ea s in g r in g s i ze . T h e CH2 b en d , CH2 wag , CH2 twi s t , CH2r o ck , r i n g b r ea th in g , an d r in g d e f o r m a t io n v ib r a t io n a ss ig n m en t a r e a l so p r e sen ted .

M I S C E L L A N E O U S A L K Y L A N D C Y C L O A L K Y L C O M P O U N DVib r a t io n a l a s s ig n m en t s f o r cy c lo a lk y l g r o u p s a r e p r e sen ted in T ab le 3 .1 0 , f or a lk y l g r o u p s o fm o n o m er s an d p o ly m er s i n T ab le 3 .1 1 , f o r cy c lo p r o p an e d e r iv a t iv e s i n T ab le 3 .1 2 , an d f o roc tadecane , oc tadecane-D38, te t racosane , and te t racosane-D50 in Table 3 .13 .

R E F E R E N C E S1. Ny quistl R. A. and Fiedler, S. L. (1993). Appl. Spectrosc. , 47, 1670.2. Ny quist, R. A. (1986). Appl. Spectrosc. , 40, 275.3. Ny quist, R. A. (1968). J. M ol. Struct ., 2, 111.4. Ny quist, R. A. and Mann, J. R. (19 72). Spectrochim. Acta, 28A, 511.5. Nyqu ist, R. A. (1967-68). J. M ol. Struct ., 1, 1.6. Ny quis t, R. A. (1984). The Interpretat ion of Vapor-Phase Infrared Spectra: Group Frequency Da ta, Philadelphia: SadtlerResearch Laboratories, A D ivision of Bio-Rad.


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Variab les in D at a I n t erpret a t ion 4 9TABLE 3.10 R a m a n an d IR da ta for compounds conta in ing cyc loa lkyl groups

C o m p o u n dRa ma nRaman Raman nea t Raman IR IR

neat neat s.CH2 str. nea t v.p. v.p.a.CH2 str. s.CH2 str. in ER. CH2 ben d a.CH2 str. s.CH2 str.

Cyclopropane (vap.)Cyclopropane

carboxylic acidChlorocyc lobutaneCyclobutane

carboxylic acidCyclopentaneChlorocyc lopentaneBromocyclopentaneCyclopen tanecarbonitr i le 1Cyclopentane

carboxylic acidCyclopentyl alcoholCyclohexaneChlorocyc lohexaneBromocyclohexaneCyclohexyl alcoholCyclohexyl amine1,2,4-TrivinylcyclohexaneCycloheptaneCyclodecaneI[CN str . , 2234 (40, p)] .

3039 (83, p) 3020 (37, p) 1452 (2, p)3020 (37, p) 1458 (5, p)

2985 (26, p) 2955 (44, p)

2943 (43, p) 2868 (45, p)2971 (46, p) 2919 (33, p)2967 (43, p) 2915 (29, p)2970 (40, p) 2875 (29, p)2967 (41, p) 2875 (29, p)2962 (84, p) 2875 (45, p)2924 (40) 2852 (52)

2940 (42, p) 2855 (45, p)2937 (71, p) 2855 (76, p)2920 (59, p)2932 (4) 2852 (4)2927 (42, p) 2853 (37, p)2914 (84, p)

2876 (21, p)

1433 (4, p)

1446 (6, p)1446 (6, p)1448 (5, p)1446 (6, p)1449 (8, p)1448 (9, p)1446 (10)

1440 (7, p)1440 (10, p)1443 (1)1441 (8, p)1442 (10, p)

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Alkyl Carbon-Hydrogen Vibrations in physics