Stone Craig avtd Thom$son 11. Vibrational Prequencies of Substituted Ammonium Ions. By P. J. STONE,'J. CYMERMANCRAIG and H. W. THOMPSON. The infrared spectra of some solid secondary and tertiary amine hydro- halides have been measured. Characteristic stretching and deformation vibration frequencies of NH,+ and NH+ have been derived and the assign- ments have been substantiated by measurements on the deuterated con-pounds. ALTHOUGHmuch is known about the vibrations of NH and NM groups in primary and and secondary amines the characteristic vibration frequencies of the substituted ammonium ions -NH,+ )NH2+ and >NH+ in the amine hydrohalides are still uncertain. Earlier evidence from the infrared absorption spectra was reviewed by Bellamy,l by Witkop, and more recently by Heacock and Mari~n.~ The last authors measured the spectra of several secondary amine hydrochlorides and derived a value close to 1600 cm.-l for a deformational vibration frequency of the NH2+ group.We have measured the infrared spectra of some solid secondary and tertiary amine hydrohalides both in potassium chloride discs and in mulls of Nujol and Fluorolube in order both to assign the characteristic vibration frequencies and also to study the band intensities since other work on the intensities of NH group vibrations in different secondary amines has revealed interesting correlations with structure.4 While it has proved im- practicable to obtain satisfactory intensity data in these cases owing to difficulties with the disc method and also because of the highly hygroscopic nature of some of the com- pounds some new frequency assignments have been deduced which are substantiated by spectral comparisons with the deuterated materials.The greater dispersion which we have used in the region 3-4 p has also enabled us to disentangle the complex situation arising from the overlapping of the stretching vibration bands of the C-H and N+-H bonds. EXPERIMENTAL The spectra were measured over the range 600-4000 cm.-l on a Perkin-Elmer 21 spectro-meter with a rock-salt prisms and the region 2000-3000 cm.-l was examined further under higher dispersion with a Perkin-Elmer 12 C instrument fitted with a lithium fluoride prism. The potassium chloride used for making the pressed discs was repurified by precipitation from a saturated aqueous solution with hydrochloric acid.It was heated in vamo at about 450" for several days to remove residual water. Additional care was necessary to prevent further adsorption of water vapour by the amine hydrohalides themselves. This adsorbed water was revealed especially by its broad absorption near 3 p and it was extremely difficult to obtain records entirely free from it. However if adequate precautions were taken the spectral records of the solids in potassium chloride discs were essentially identical with those obtained by using mulls in Nujol or Fluorolube. The compounds were specially prepared and the following recorded m. p.s indicated satis- factory purity diethylamine hydrochloride 223-5" dibenzylamine hydrochloride 255-256" piperidine hydrochloride 242" piperazine dihydrochloride 322-323" morpholine hydro- chloride 175-176O thiazine hydriodide 156-157" N-methylpiperidine hydriodide 121-5" N-benzylpiperidine hydrochloride 176-178" benzyldimethylamine hydrochloride 173-1 74" N-ethylmorpholine hydriodide 147-148" N-benzylmorpholine hydrochloride 243-244" N-benzyl-N'-methylpiperazine dihydrochloride 257-258" N-benzylpiperazine hydrochloride 167-S-168".Deuteration was carried out as follows. The amine hydrohalide was placed in a small glass tube attached to a vacuum line and adsorbed water was removed by prolonged evacuation. Dry air was then admitted to the line and 99% deuterium oxide added to the solid from a Bellamy " The Infra-red Spectra of Complex Molecules," Methuen London 19b4.Witkop Experientia 1954 10 420. Heacock and Marion Canad. J. Chem. 1956 34 1782. Russell and Thompson J.,1955 483; Proc. Roy. SOC.,1956 A 234,318. [19581 Vibratioutal Frequencies of Sivbstituted Ammonium Ions. dropping funnel sealed to the apparatus. After an interval the heavy water was pumped off; this process was repeated several times. A high degree of deuteration was thus achieved and although it was incomplete the marked changes of intensity of certain bands could be used as a guide to the assignments. RESULTS In the spectra of the secondary amine hydrohalides there are two regions of particular interest. One includes a complex pattern of bands between 2500 and 3000 cm.-l and the other lies between 600 cm.-l and 1600 cm.-l.The former includes the stretching vibrations of the substituted ammonium ion and the latter its deformational modes in addition to vibrations of the whole skeleton. After deuteration a new group of bands appears between 1900 cm.-l and 2300 cm.-l in which the N+-D stretching vibration bands are present and there are other new bands around 1100 cm.-l and at lower frequencies. Simultaneously with the appearanc; of these new bands there are changes in the spectra near 3000 cm.-l and 1600 cm.-l as compared with those of the undeuterated compounds. Both before and after deuteration the spectra are very complex with many of the strongest bands appearing to consist of several components and it is quite impossible to make many direct assignments of vibration frequencies.There appear to be many combination or difference bands which cannot yet be interpreted. The situation is complicated further by the possibility of hydrogen-bond formation some of which may arise from the presence of residual water in the solids. However the results obtained upon deuteration lead to a few definite assignments. First with the secondary amine hydrohalides containing the >NH2+ ion there are two N-H stretching vibration bands the symmetric and antisymmetric vibrations at about 2800 cm.-l and 2920 cm.-l respectively which decrease in intensity upon deuteration. Two new bands then appear near 2120 cm.-l and 2200 cm.-l. The results are given in Table 1 and the positions quoted for the bands should be regarded as correct to within 10 cm.-l but cannot be fixed more precisely because of their broad nature and overlap with other bands.In the case of piperazine dihydrochloride the assignments are doubtful owing to an unusual complexity which was also noted by Heacock and Mari~n.~ TABLE1. >NH,+ stretching vibrations. Asymmetric Symmetric < h \ v(H) v(D) Ratio v(H) v(D) Ratio Diethylamine hydrochloride ............ 3915 2195 1.33 2824 2100 1.34 Dibenzylamine , ............ 2920 2212 1.32 2789 2112 1.32 Piperidine ## ............ 2920 2170 1-35 2844 2117 1.35 I1 Morpholine ............ 2920 2220 1.32 2780 2108 1.32 Thiazine hydriodide ........................ 2920 2223 1.32 2820 2164 1.30 Piperazine dihydrochloride ............... 2950? 2281? (1.29) 2840? 2093 (1.36) Table 1 includes the frequency ratios for the isotopic species.If the N+-H link is regarded as the vibrating molecule the ratio should be close to 1.37 and Edsall found values of about 1.36 for the vibrational frequencies of the NH3+ion from Raman spectral measurements on methylamine and hydrazine hydrochlorides The ratios now observed therefore support the assignments suggested. Near 1600 cm.-l there is another sharp band of the secondary amine hydrohalides which is replaced by another around 1200 cm.-l after deuteration. The results are given in Table 2. This band is clearly due to a deformational mode of the ion and supports the assignments made by Heacock and Marion. Indeed the band of amino-acid hydro- chlorides in this region discovered earlier by Randall et aZ.6 can now be definitely associated with the )NH,+ group deformation.The new point to note is that its exact location Edsall J. Chem. Phys. 1940 8 520. I‘ Randall Fowler Fuson and Dangl Infra-red Determination of Organic Structures,” van Nostrand New York 1949. Vibrational Frequencies of Substituted Ammonium Ions. depends upon the particular residues attached to this group. There is in fact a rough correspondence between the value of this frequency and the Q* values of the attached groups discussed by Taft.? As Q* increases the frequency is lowered. Thus with the TABLE2. >NH,+ Deformational mode. v(H) v(D) Ratio Diethylamine hydrochloride .............................. 1600 1178 1.36 Dibenzylamine , ..............................1572 1160 1.36 Piperidine .............................. 1598 1240 1-29 I* Morpholine .............................. 1580 1275 1.24 I1 Thiazine hydriodide .......................................... 1560 1140 1.37 Piperazine dihydrochloride ................................. 1591 - N-Benzylpiperidine hydrochloride ........................ 1597 - hydrochlorides of N-phenylglycine diphenylamine dibenzylamine N-ethylaniline B* dimethylamine and diethylamine for which = 1-65 1.20 0.45 0.50 0 and -0.2 the frequencies are 1560 1572 1572 1589 1600 and 1600 cm.-l respectively. In the spectra of the secondary amine hydrohalides there are two other prominent bands near 2500 cm.-l and 2400 cm.-l which decrease in intensity after deuteration.These must be connected in some way with the >NH,+ group and as Table 3 shows there is a regular relation between these bands and those assigned above to the stretching vibrations of this group. The interpretation of this pair of bands is uncertain. They could arise as difference bands caused by absorption from a low-frequency fundamental near 400 crn.-l or alternatively as vibrations of the >NH2+ group lowered by hydrogen bonding either with other similar groups or even with residual water in the solid. TABLE3. Va Venn A vb Vaatiaym A Diethylamine hydrochloride ..................... 2390 2824 434 2483 2915 432 Piperidine ..................... 2427 2844 417 2496 2920 434 D# Morpholine ..................... 2461 2780 329 2605 2920 315 I, Thiazine hydriodide .................................2400 2820 420 2484 2920 436 Piperazine dihydrochloride ........................ 2426 2840 424 2515 2950 435 With the tertiary amine hydrohalides an assignment of the >NH+ group stretching vibration has been made as given in Table 4. The frequency ratio for the isotopic species is between 1-25 and 1.30. These results agree with earlier suggestions by Lord and Merrifield * for triethylamine hydrochloride and pyridine hydrochloride also included in Table 4. TABLE 4. >NH+ Stretching vibration. v (HI v(D) Ratio N-Methylpiperidine hydrochloride .............................. ca. 2680 ca. 2068 1.30 N-Ethyl morpholine ) .............................. 2665 2046 1.30 Benzyldimethylamine , .............................. 2630 2023 1.30 N-Benzylpiperidine * .............................. 2512 1962 1.28 N-Benzylmorpholine .............................. 2480 1980 1.25 N-Benzyl-N'-methylpiperazine dihydrochloride ............ 2330 1850 1.26 Triethylamine hydrochloride .................................... 2540 1965 1-29 D# Pyridine .................................... 2425 1880 1.29 This work was supported by a grant from the Hydrocarbon Research Group of the Institute of Petroleum whom we thank. PHYSICAL LABORATORY, CHEMISTRY OXFORD. [Received July 3rdD 1957.1 See Newman " Steric Effects in Organic Chemistry," Wiley New York 1956. 8 Lord and Merrifield J.Chem. Phys. 1953 21 166.